Brakes on my 72 F250 worked excellent from factory. Dual piston front discs, big drums in rear, dual diaphragm vac booster, 32" tall tire, worn out 390 2bbl.
Fast forward to 1994 F350 axle swap, mild 460, 40" tires. Brakes are essentially the same, dual piston front discs, big drums in rear. Same booster, same master cylinder. Performance significantly degraded. Part of this was lower engine vacuum level (10-12"). The big tires directly reduce braking torque.
So hydroboost it is
Ford hydroboost units are harder to find than the chevy favor.
If you buy a renam, they do not come with the booster to master cylinder push rod, spring or clip. The length of this pushrod is vehicle specific.
Ford SD unit
So that leaves a few options
- junk yard unit {perhaps just for core and MC push rod)
- reman HB plus rod (if you know what lenght you need, and can find one to buy)
- New ford Motorcraft, complete.
I found a Chevy HB in local JY for $80. The pedal rod was significantly shorter. The MC rod was also significantly shorter. The pedal rod bore size is larger and would require a different bushing. The nitrogen accumulator was damaged and loose.
Ford vac booster pedal rod length = 9-1/8"
Chevy HB pedal rod length 5-3/4"
difference = 3-3/8"
Ford booster spacer 3-3/8" (magic?)
Ford MC rod length = 1" protrusion from face
Chevy MC rod length = flush to face
Ford MC stud spacing = 3-3/16"
Chevy MC stud spacing 3-1/4"
So several differences including mounting plates. This unit could be made to work. It would not allow a spacer on firewall, and rod will be short by mount plate thickness, which is OK and would move pedal height down, relocate brake switch.
Whould need to use a Chevy MC. A ford MC could be used, but would need mount holes hogged out and a longer MC push rod, or a rod spacer/extention. Also have to get a different nylon pedal bushing.
I choose to return the junk unit and buy a Motorcraft BRB33. This is for a 99 up super duty, although there are more than one flavor of those too. This gets me the rod lengths I wanted, correct pedal bushing bore, and a new high quality unit.
BRB33 pedal rod length = 7-1/4"
Firewall spacer needed 1-3/8"
MC rod length = 1"+ and square end
So I cut a rectange and drilled some holes to make a custom mount plate out of 3/16 plate. This allowed me to use factory captive nuts on pedal assembly, and some 2-1/2 3/8 studs. Quick and easy, no mods to truck.
To work with 1972 MC, the push rod is 0.060" too long. Easy enough to modify, once you figure out how to remove the retainer...
To work with a 1994 F-350 MC (BRMC38), the end just needed to be radiused. Pop it in your lathe...
Orientation/clocking
HB units can be installed at any angle, but there is an 'up', as installed by the OEMs, hoses on top. The mustang 4.6L is at roughly 90* and has a special MC with mount hole vertical, accumulator up. The older models have a drain channel on the 'bottom', newer ones do not have one.
I test fit the unit upside down. return lines pointed at header. looks kind of goofy, puts spool valve underneath, opposite of MC reservoir.
Fittings
Newer (90s up) HB units have an 18 x 1.5 mm o-ring inlet and a 16 x 1.5 mm o ring outlet. The two outlets are on the same side. Older have inverted flare. Most convert to AN6, so the metric to AN adapter are easy to find. I would only use steel fittings for this application.
Hoses
Bumps have a 3/8 IF pump outlet and a 5/16 IF P/S box inlet. Some use IF to AN adapter and just AN6 to AN6 hose. I used 90* IF hydraulic hose ends and 90* AN6. My hoses are about 32" long. I bought hose ends and bulk hose to make custom length hoses, with proper end fitting clocking (due to the 90s).
Another option is field serviceable hose ends and matching Teflon lined hose. They are expensive and odd fittings (like 5/16 IF 90) are hard to find. There are both aluminum and steel version, suggest only steel. I could not find all the pieces in stock of any given product family at the time.
After test fit I adjusted spacer length to get pedal in same location so I would not have to move the brake light switch. I also added booger welds as clocking keys to the HB bore like the OEM plate.
differences in super duty vs bumpside mount plates.
I went with a new dual return pump reservoir for a few reasons. ACDelco 19207054. New one is intended to be mounted at an angle, but with cap level. The offset design allow more clearance to A/C pump to get belt on/off. Fits perfect. I was able to run a 1/2 shorter belt and get pump more vertical. This solved interference problem of fluid cooler return lines also.
After fitting hoses, I had them crimpled at local shop. Installed existing MC at this time with some washers to space it out. One change at a time.
I was going to bend hard lines off of HB and use shorter hoses, hidden down by steering shaft. This puts them near header and I was worried about heat transfer. So I kept things simple and just uses hoses with 90s and ran them forward, direct to pump/box.
So, how does it perform? Sensitive. Only needs light pressure to stop now. Any more locks up rear brakes. Guessing 2x-5x the assist level over vaccum setup.
I put some miles on it and it's going to be OK. One cheap AN to metic fitting is leaking. MC is still leaking badly.
Next step is the new Ford motorcraft MC, BRMC38, to match the axles I have. Rear circuit will get adjustable proportioning valve and a #10 residual.
ignant???
