Booster Questions
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Senior User
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From: Graham, Wa.
Booster Questions
I have completed my conversion to discs front and back on my 74' and also installed a new master cylinder from a 78' (disc/drum version) and a wilwood adjustable prop valve and I still have somewhat spongy break pressure. I have bled the damn thing a hundred times, but the only item that I did not change on the swap was the booster as it looked pretty new (PO must have changed it recently). Is there a difference between the booster for a 4 wheel drum system vs. the 4 wheel disc system?
Lead Driver
Joined: Mar 2004
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The pedal should be rock hard with the engine off. Any spongy feeling could be caused by air in the system. Did you change the prop valve ? If you did is the pin in all the way ? Are the rear brakes adjusted up.
Cargo Master
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Yes correct. I meant to say with the engine running.
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From: Graham, Wa.
Actually the pedal is rock hard with the engine off, but gets spongy with the engine running and there are no brakes to adjust given that they are all discs. There is not any sort of "metering" valve in the Wilwood adjustable prop valve that I need to be tinkering with when I bleed the lines is there?
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Post Fiend
Joined: Mar 2011
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From: Lynchburg, VA
So your brakes work, the pedal is hard with the engine off, and theres a bit of sponge of in the pedal with the engine running. i dont see the problem. My pedal doesnt exactly feel like a rock.
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Senior User
Joined: Jul 2006
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From: Graham, Wa.
So I have been living with these spongy brakes for a couple of months and I am not content with that. I would like to find an appropriate booster for the disc/disc setup. Can anyone suggest an appropriate booster for this conversion? Need to keep it small as I don't have much room between the valve cover (dual diaphragm?)
I have been looking through Cardone's site and can't figure out which ones are singles and which are doubles.
I have been looking through Cardone's site and can't figure out which ones are singles and which are doubles.
Hotshot
Joined: Feb 2004
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From: The Shenandoah Valley
Remove the booster, install a non power brake master cyl .... and you'll have hard brakes. Hard pedal and hard to press.
Even my several CVPI police cars when new had what some would call "spongy" brakes when engine was running and I was sitting still pressing on the brakes ...
... but in use you don't press that hard to stop.
What master cylinder and what brake booster are you running now? You can't just mix and match.
The below is from an article dealing with 78-79 Broncos and F-150s but you may find something in it that will help.
You might need to change to 78 or 79 mounting? 
At least the part numbers will give you a start point.
Even my several CVPI police cars when new had what some would call "spongy" brakes when engine was running and I was sitting still pressing on the brakes ...
... but in use you don't press that hard to stop.
What master cylinder and what brake booster are you running now? You can't just mix and match.
The below is from an article dealing with 78-79 Broncos and F-150s but you may find something in it that will help.
Project Code Name "Double Agent" F-350 Brake Upgrade
The '78-'79 Ford F-350 tandem booster (dual 9 1/4" diaphragm) is a direct bolt in that requires only a brake line modification. If you own a 78 Bronco will need the booster from a '78 F350 4WD or 2WD. If you own a 79 Bronco then you need the booster from a '79 F350 2WD only! Also, the '79 F350 with cruise control was yet a different booster. You can get the booster/master cylinder combo at most auto part stores. The largest remanufacturing company is A-1 Cardone and it's part #50-3311 for a 78 model, and part #50-9337 for a 79 model (without cruise control). Those are the p/n for the booster/mc combo. The only change needed here is replacing the line nut fitting on the chamber for the front brakes. The Bronco master cylinder uses a 9/16"-18 threaded nut and the F350 master cylinder uses a 3/8"-24 threaded nut. You must also twist your lines around to the other side of the master cylinder because the F-350 has the fittings on the opposite side. Just find a local shop to double flare your brake line with the new fitting and your done. At this point, if you add the F-350 rear wheel cylinders your brakes will show a major improvement and it is not absolutely necessary to add the Thunderbird calipers, but it is easy enough and will give you even more braking ability.
The '78-'79 Ford F-350 tandem booster (dual 9 1/4" diaphragm) is a direct bolt in that requires only a brake line modification. If you own a 78 Bronco will need the booster from a '78 F350 4WD or 2WD. If you own a 79 Bronco then you need the booster from a '79 F350 2WD only! Also, the '79 F350 with cruise control was yet a different booster. You can get the booster/master cylinder combo at most auto part stores. The largest remanufacturing company is A-1 Cardone and it's part #50-3311 for a 78 model, and part #50-9337 for a 79 model (without cruise control). Those are the p/n for the booster/mc combo. The only change needed here is replacing the line nut fitting on the chamber for the front brakes. The Bronco master cylinder uses a 9/16"-18 threaded nut and the F350 master cylinder uses a 3/8"-24 threaded nut. You must also twist your lines around to the other side of the master cylinder because the F-350 has the fittings on the opposite side. Just find a local shop to double flare your brake line with the new fitting and your done. At this point, if you add the F-350 rear wheel cylinders your brakes will show a major improvement and it is not absolutely necessary to add the Thunderbird calipers, but it is easy enough and will give you even more braking ability.
At least the part numbers will give you a start point.
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My truck is a F150 with F250 axles front(44) rear(60). The front is a duel piston caliper and the rear is a disc conversion kit using Chev k20 single piston calipers. I'm still using my old stock brake booster and prop valve. Running 36/13.5/16 IROK and having no problems stopping. Have you tried using a brake bleeding kit and also if the bleeder valve is below the caliper reservoir it will trap the air in the top of the reservoir. If thats the case you have to pull the caliper and bleed them.
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From: Graham, Wa.
All my bleeder valves are at the top of the calipers, so I can't imagine that is the problem. I have noticed that the original booster, which appears to be a dual diaphragm, constantly has oil/brake fluid at the bottom of it, although it is not dribbling down from the M/C. It seems to be weeping from someplace.
I swapped out the p-valve with a Willwood that has a P-valve for the rears only, and it is my understanding that these do not have a "metering valve" that needs to have a pin moved to bleed. If I am incorrect here, someone please let me know.
Thanks for all the info guys.
I swapped out the p-valve with a Willwood that has a P-valve for the rears only, and it is my understanding that these do not have a "metering valve" that needs to have a pin moved to bleed. If I am incorrect here, someone please let me know.
Thanks for all the info guys.
Postmaster
Joined: Oct 2010
Posts: 4,656
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From: Northern and Southern Cal
Just a thought.
This master cylinder may be your problem.
You have a disc/disc set up.
Disc brakes require more fluid volume to operate. The fact that you used a disc/drum master cylinder may be the reason for the spongy pedal.
I don't know for sure but it's worth a look.