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1937 Plymouth Tudor Vacuum Boost to Hydraulic Brake Booster Conversion

My car has a 2003 Dodge 5.7 HEMI engine, an under floorboard brake system with a seven inch dual diaphragm vacuum brake booster, a 1 inch bore dual port master cylinder, 1988 Camaro front disc brake, 1978 Lincoln Versailles rear disc brakes and 30 inch diameter rear tires. The car occasionally tows a 1300 pound trailer. The braking power of this configuration was unsatisfactory.

After consideration of several options, conversion to hydraulic brake boost (hydra-boost/ Hydraulic brake booster/HB) was selected.

I went on-line to do research, then to ebay and ended up contacting seller “sterlingworth16”. The seller turned out to be Elliott Sydnam of Custom Hydraulic brake booster. I explained to Elliott my brake issue and how space was a limiting factor for this retrofit. To my pleasant surprise Elliott was a wealth of knowledge and was eager to help me select the appropriate HB for my car. I discovered Custom Hydraulic brake booster could set me up with just about any adaptor you could imagine to install HB on my vehicle. Custom Hydraulic brake booster makes a wide variety of adaptors, tools and booster related items as well as maintains a variety of Hydraulic brake booster (HB) units in stock. Check out his web site CustomHydraulic brake It is full of helpful information. After a couple of question and answer sessions with Elliott he sent me a unit specifically configured for my application and a packet of installation information.

Elliott pointed out my power steering pump was under sized for this application and I would need to increase my pump pressure. Elliott sent me the appropriate pump spring and follow control valve to modify my existing pump but I later opted to change pumps for reasons I will cover later.

Elliott sent me a new Bendix unit commonly used on the 1984-87 GM Military CUCV (Commercial Utility Cargo Vehicle) M1008. The trucks were all rated as 1-1/4 ton. It turned out this new HB would only be 3/4 of an inch longer than my vacuum booster.

I next roughed out a project schedule
  1. Prep car
  2. Vacuum test and Check Valve inspection
  3. Baseline brake pressure
  4. Fit check and retrofit the booster assembly
  5. Remove front fenders, inner funder panels, hood and radiator nacelle
  6. Retrofit power steering pump
  7. Plumb hydraulic lines
  8. Bleed system
  9. Reassemble front fenders, inner funder panels, hood and radiator nacelle
  10. Re-baseline brake pressure
  11. Road test and adjust proportioning valve

1. Preparation
Set car on jack stands approximately 20 inches high. Removed all four wheels and snugged two lug nuts per hub in order to stabilize the rotors.

2. Vacuum Test
I ran the engine at idle. Engine vacuum at the manifold read 18 inches. I tested the vacuum booster check valve and found it was operational and not leaking.

3. Baseline brake pressure
I performed a baseline brake fluid pressure test at front and rear calipers using a brake pressure gauge. I could not come up with a way to measure the force being applied to the brake pedal for consistent repeatability of the test so I applied what I felt was maximum force seated in normal driving position. I then ran a series of tests to establish a norm. Eight out of the ten readings were within 100 pounds and I recorded that value.

Front caliper, no boost (engine off): ~500 PSI, with vacuum boost at engine idle: ~1000 PSI

Rear caliper, no boost: ~500 PSI, with vacuum boost engine at idle: 800 PSI

4. Fit check and retrofit the booster assembly

I removed the adjustable proportioning valve which is mounted to the frame just below the master cylinder (MC). I was then able to remove the vacuum booster and MC assembly from under the car. I would reuse my 1 inch bore MC so I reattached it to the new HB unit and attempted to reinstall the assembly back into the car. When I tried to install the assembly in the upright position, the HB accumulator hit the inside frame rail and one of the frame crossmembers interfered with the reservoir inlet fitting of the secondary MC port. I rotated the assembly 90º counter clockwise and everything cleared. I tried the two other positions but this orientation appeared to be the best. The sideways orientation of the MC inlet ports seemed odd at first but with a remote reservoir configuration the MC does not care what position it is in as long as it gets fluid. Modifying the frame crossmember was not an option. I was not concerned about removing air from the MC as long as I pressure bled the MC through the inlet port hoses at the remote reservoir located on the firewall. I then bolted the assembly in place to fit check the remaining components. I discovered the reservoir hoses for the MC were now too short, the clevis on the input rod was too long because the jamb nut could not be installed on the input rod, and there would be very tight plumbing space issues (that was expected). I also discovered the bottom MC mounting stud obstructed the proportioning valve knob when I installed the proportioning valve back onto the frame. It was also obvious I would need to fabricate new MC plumbing due to the added length of the HB moving the MC rearward 3/4 of an inch and the new sideways orientation of the MC.

The short reservoir hose was an easy fix. I connected the longest hose to the primary MC port and reused the ends off of the shorter hose to make a new longer hose for the secondary port. The input rod clevis was too long and I was not able to fit the jamb nut Elliott sent with the unit. There was about one inch of thread in the clevis body so I decided to shorten it by 1/2”. This would allow proper pedal adjustment and the incorporation of the jamb nut. In order for the jamb nut to seat properly against the clevis body I need to cut the clevis off flat and square. I found a three inch long bolt with 3/8-24 treads and cut the head off. I chucked it in my drill press and threaded the clevis on from the pin side. I turned the drill press on and cut ½ of an inch off using a die grinder fitted with an abrasive cut off wheel.

I removed the bottom MC mounting stud from the HB after much time and effort. I could not get the metric stud to back out. I finally sent Elliott an email asking for advice. He replied right away with the solution…They seat with left hand threads!...Duh! After I removed the stud I replaced it with a 3/8-24 grade eight bolt, two washers and a fiber lock nut. I inserted the bolt from the rear and fastened the nut from the front. This configuration rendered the space necessary to allow operation of the knob on the proportioning valve again.

I then fabricated a new brake line to run from the front brakes residual valve to the primary port of the MC and a new line from the proportioning valve (rear brakes) to the secondary port of the MC. Note: Residual valves are used when the master cylinders are mounted below the level of the calipers. This keeps the brake fluid from draining back to the master cylinders so you don't have excessive pedal travel and caliper restriction. They are available in two sizes, 2 lb. and 10 lb. The 2 lb. is used for disc brakes and 10 lb. valves are used in drum brake setups. The higher drum brake pressure valve is necessary to compensate for return spring tension.

I use 7 gauge galvanized wire to mock-up my bends and then I follow my mockup when bending the tubing for an accurate fit.

5. Remove front fenders, inner fender panels, hood and radiator nacelle
In order to access and remove the power steering pump it was necessary to remove the body work from the front of the car. The length of the engine and transmission package left little room between the radiator and engine when I built this car.

6. Retrofit power steering pump
I originally constructed the car with a General Motors Type III power steering pump to accommodate the low flow and pressure requirements of Ford Thunderbird rack and pinion steering setup. The Type III pump was fitted with a custom serpentine pulley to make the pump compatible with the OEM Dodge serpentine pulley system. I now converted back to the original OEM Dodge pump which has a larger reservoir, three point mounting instead of the GM two point mounting and a non-custom OEM pulley that is available at most parts stores. I wanted to keep as many parts as possible OEM because I have found from past experience using aftermarket parts create problems when future repairs or especially repairs on the road are necessary. The Dodge pump was sent out for a rebuild and to have the flow adjusted to two gallons per minute for the Ford Thunderbird steering and the pressure lowered from the original Dodge specs of 1615 PSI to 1450 PSI for the new Bendix HB. The new HB unit is rated for 1500 PSI but it has no relief valve so I opted to run a conservative pump pressure to avoid pressure leaks or damage to the system.

My under floor brake system is configured so the master cylinder and booster face the rear of the car instead of the front of the car. In vehicles where the master cylinder and booster are mounted to the firewall, the master cylinder and booster generally face the front of the vehicle. This rear facing configuration created some plumbing challenges due to the limited space within the frame rails, crossmember and floorboard.

7. Plumbing hydraulic lines
The Bendix HB unit was designed to use 5/16 -16mm x 1.5 and 3/8 -18mm x 1.5 O-ring or “Bump Tube” style hose ends. The 3/8 -18mm port is the pressure supply inlet from the power steering pump, the 5/16 -16mm port is the pressure supply outlet which flows fluid to the power steering gear box or rack pressure supply inlet. There is also a 5/16-24 thread x 3/8 hose barb fitting which serves as the power chamber vent line. The power chamber vent line returns fluid to the power steering pump reservoir when the brake pedal is released after each application.

These hoses with “Bump Tube” ends work well in OEM applications but where custom installations are needed these ends are not friendly to fabrication. I was not able to find a tool to make bump tubes and I was not able to find bump tube hose ends sold separately other than a few air conditioning pieces and they were relatively expensive compared to other options. I assume one option would be to buy wrecking yard hoses and cut the ends off and do flare and unions or weld them to your plumbing but I opted out of that approach. I finally decided to use AN or JIC style fittings since I had used these fittings in other systems on the car and many adaptor and fitting options were available on the internet and through local suppliers. The following examples are some other plumbing options I found:

Flare inserts. These can be used to convert a typical bump-tube style ports to be used with flare style fitting. Hydratech sells a set (part number HBS9106). Set includes one 5/16 Tube Seat and one 3/8 Tube seat. Flare Inserts can also be found at Autozone and air conditioning fitting suppliers.

Another option is -6AN to bump- tube adaptor fittings. Sweet Performance Products, Inc. 714-223-9322 makes a Hydraulic brake booster fitting kit (Product Code: HYDD246). The kit includes, one -6 to M16-1.5 bump-tube fitting, one -6 to M18-1.5 bump-tube fitting and one -6 to 5/16-24 fitting.

Other options are high pressure hose kits. I found several companies who make Hydraulic brake booster hose kits.

Yet another option is steel tubing. Since all of the tubing on my car is stainless steel I did not investigate mild steel tubing pressure ratings. I would not hesitate to use mild steel tubing on another project as long as I verified the pressure ratings first. The tubing I looked at was 304/304L Stainless Steel Tubing - 3/8" OD x 25' .020 wall.

In order to use tubing with AN or JIC fittings you must first flare the end of the tubing to 37º and use a sleeve and tube nut.

Common 45º flaring and double flaring tools are made for Standard brake and fuel lines will not work with AN or JIC fittings. AN and JIC fitting ends are machined to 37º and therefore require a matching 37º flare on the joining tube.

I decided on the stainless steel tubing option with the bump-tube to AN adaptors. I did consider hoses but tight turns were a possible kink or flow issue for my particular installation. I also was concerned about long runs of hose and the diameter of the hose passing through the available narrow routing passages to the front of the car. I purchased the tubing off of ebay and the adaptors from Sweet Performance Products.

After the adaptors arrived and were installed it was apparent this option would not work due to the height of the fittings and the space restrictions within the frame rails and the floorboard. I could not make 90º bends short enough with the tube nut and sleeve in place behind the flaring tool. My finished connections would be a minimum of three inches high. My next option was to use banjo fittings. I went on-line and found 16mm and 18mm Banjo Fittings with -6AN male connections. I purchased them through Jegs. (800)345-4545. The item numbers were 8021560 for the 16mm fitting and 8021570 for the 18mm fitting. The swivel feature made connecting tubing easer and was good for tight low profile areas but I later found the rotation was limited on the 16mm pressure return fitting due to the body design of the HB and the slender design of the banjo fitting. When the tube nut was installed on the banjo it was wider than the body of the banjo and would collide with the surface of the HB unit. I was not able to space the banjo up due to the limited number of threads on the banjo bolt. This issue limited the rotation of the banjo to approximately 130º to the rear. Note this would not be an issue on firewall mounted or forward facing installations.

I also need a low profile piston vent fitting. I ended up fabricating an elbow from brass fittings I had on hand. I started with a 1/8 pipe thread (NPT) to 1/8 tube compression fitting. The compression cap threads were 5/16-24 which would thread into the HB body and the other end was male 1/8 NPT. I then took a 3/8 hose barb elbow fitting with 1/8 male NPT threads and cut the male NPT threads off. I drilled the hose barb body with a 21/64 bit and taped the hole to 1/8-27 NPT. I then examined the bore in the hose barb that came with the HB and discovered it had a 1/8 inch bore. The compression fitting I was using for the base of my elbow had a smaller bore so I drilled it out to 1/8 inch to match the original HB fitting. I wanted to assure there would be no flow issues. I assembled the elbow on the HB unit mounted in the car so I could tighten the pipe threads to a point where the hose barb was pointing in the direction I wanted the return plumbing to go. I would later add an O ring upon final assembly to assure a proper seal at the HB body.

It was now time to fabricate the HB hydraulic lines. I cut a five foot piece of 7 gauge mockup wire and began laying out the pressure supply line. I then bent the tubing to shape, flared the HB end and connected it to the banjo fitting on the HB. I then repeated the process for the pressure return and the power chamber vent line. Due to the tight clearances above and on the right of the HB I ran all of the tubes rearward then made 180⁰ bends to go back forward close to the right side of the HB body and through an existing slot in the HB/brake pedal mounting bracket.

I left them long and devised a way to bundle and support them to the frame.

I had connecting points for the power steering pump and the steering rack but I did not have a point for the reservoir return. All of the installation instructions I found recommended a reservoir with two independent return inlets, one for the steering and one for the HB. My reservoir only has a single 5/8 inch hose barb return. The second choice was to use a Tee fitting. Many articles I read cautioned against Tee fittings due to possible pulsing or other anomalies when applying brakes and turning at the same time. A third option would be a Y fitting. Several manufactures make Y fittings but they were not available in a configuration I could easily use. Because my OEM Dodge reservoir has a 5/8 inch return inlet, I was confident it could handle the flow of two 3/8 return lines so I opted to make my own full flow Y fitting. I salvaged a brass heater core inlet tube and two 3/8 steel tube bends from a scrap power steering hose. I cut the two 3/8 steel tubes to length and inserted needle nose Vise-Grips inside the tube openings and clamped them together. I then held them in place under the reservoir return inlet and adjusted the tubes until they were pointing in the direction I wanted. I took them to the bench and tack welded them together. I then cut the brass heater core tube to length and stretched the end opposite the barb to an oval shape and brazed the two 3/8 steel tubes to it. I cut a two inch piece of hose and joined the Y fitting to the reservoir inlet.

I then began connecting my HB tubing to the respective points. The diagram below illustrates how the hydraulic circuit is connected.

The Thunderbird rack was also designed to use bump-tube style fittings. My steering rack was converted to bump-tube to -6 AN male adaptors during the original construction of the car. I needed to install a 16mm to -6 AN adaptor on the new power steering pump but the shoulder on the OEM outlet fitting was too deep and it would not allow the O ring on the adaptor to seat. I removed the OEM fitting from the pump and chucked it in my drill press, turned it on and cut ¼ inch off using a die grinder fitted with an abrasive cut off wheel. I cleaned and reinstalled the fitting into the pump and installed the AN adaptor fitting on to it. I then took measurements for the two pressure hoses. Each would need to be fitted with a 90⁰ full flow fitting on one end and a straight fitting on the other. I discovered it was less expensive to use crimp on fittings rather than the field serviceable type so I measured and had the two Teflon/steel braded high pressure hoses made at a local industrial supply center. The low pressure return hoses I fabricated from 3/8 I.D. oil resistant hose typically used with transmission coolers. I flared the pressure supply and pressure return tubes coming from the HB connected them to the pump and steering rack using -6 AN unions. On the steering rack low pressure return line and the HB vent line I used spring clamps to secure the hoses to the tube and fittings.

I ran hoses to the engine and rack to provide movement and vibration protection. Notice the loop on return hose rather than short straight run. I then snugged the banjo fittings on the HB, disconnected the tubes and removed the HB from the car. On the bench I marked the HB banjo fittings so I could reinstall them in the same position. I removed the banjo bolts and applied blue Loctite to the threads and reinstalled them. I torqued the 16mm banjo bolt 40 foot pounds and the 18mm bolt to 45 foot pounds. The brass piston vent fitting I gingerly torqued to 10 foot pounds.

I installed the dust cover over the input rod, added the jamb nut and clevis then reinstalled the HB and MC back into the car for final assembly. I connected and adjusted the clevis for proper pedal height, and connected the brake light switch spring to the pedal arm. I reconnected tightened all of the hydraulic and brake lines and torque the MC banjo bolts to 25 foot pounds. I pressure bleed the brake lines and refilled the MC remote reservoir.

8. Bleeding the system
To bleed and prime the HB and steering system I disengaged the serpentine belt from the power steering pump and filled the pump reservoir with fluid. I then turned the steering wheel from lock to lock four or five times. I then used a low speed drill motor fitted with a 5/16 ball hex inserted in the pump shaft to spin the power steering pump clock wise. As I spun the pump I watched the fluid in the reservoir as it bubbled at first then turned fizzy for a few seconds and then ran clear after about one minute of priming. I then turned the steering wheel lock to lock a couple more times and spun the pump with the drill again, no air…good!

9. Re-assembly
I reassembled the front end of the car and ran the engine. I checked all fitting joints for leaks, cycled the steering several times and applied the brakes and looked for leaks again, all was good. I shut off the engine and checked the steering and brake fluid levels.

Note: In some cases where leaking joints are a problem you can use copper inserts that go between the fitting and the tube flare called Flare Savers or Flare Inserts. They act as a gasket and will usually stop leaks. 3/8" Flare Savers or -6 AN Flare Insert

10. Re-baseline the brakes
Next I installed the brake pressure gage on the front caliper and began my before and after comparison test. The front caliper with Hydraulic brake booster and engine at idle: ~2400 PSI

Rear caliper with Hydraulic brake booster and engine at idle: 1600 PSI

Front caliper pressure before Hydraulic brake booster was 1000 PSI. Pressure with Hydraulic brake booster is now 2400 PSI; yield 140% increase. Rear caliper before Hydraulic brake booster was 800 PSI. Pressure with Hydraulic brake booster is now 1600 PSI; yield 100% increase. Note the adjustable proportioning valve setting was not changed between the before and after tests to assure consistency. Changing the proportioning valve will shift more pressure to the rear if needed when the car is road tested for braking balance.

11. Road test and adjustment
I finished assembly of the car prepared for the road test. Road test achieved four wheel lockup balance with the proportioning valve set at one turn open. The new HB system has phenomenal braking force with the eight inch wide tires in front and eleven inch wide tires in rear. The pedal feel and control is very good, not under responsive or over sensitive.

I highly recommend Hydraulic brake booster systems to people who want or need higher performing brake systems on their vehicles.

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