Another power brake conversion question

[MyBronco15]

So, I installed a power brake conversion kit in my 71 with stock drum brakes and H block. As far as I can tell there is no air in the system but the pedal is indeed very soft the first couple inches then very hard. The brakes do work but not like they should, indknt believe anyway.

I took some.measirements to be a little more specific. The pedal sits 6 11/16 inches off of the floor now. Ot moves one half of an inch with no pressure I assume where the booster plunger engages master cylinder. Looks like I can take that out about 1/8 to adjust that no problem.

However, then you can feel the pedal slightly engage but pushes freely 1 7/8 inches with almost no feedback. Once it hits that plateau, brakes are very firm and stop well but pedal is very hard.

I had my seven year old son pump the brakes for me with the engine on and during that intial 2 inches of pedal the MC shoots fluid up and out of the resevoir and thenengine idle will change some but other than that things look right.

What do you think???

 

[Pops68]

Did you bleed your brakes??? You didn't say whether you did or not.

 

[MyBronco15]

Yes, of course I bled my brakes. Bench bleed and full bleed/flush with speed bleeders. I guess I didn't specifically bday they were bled, just said i didnt think there was any air. I also adjusted shoes to get rid of any extra play.

 

[suckerpunched]

did the master cylinder come with the kit? do you know anything about it, like bore size? was it made to be used on a drum/drum system?

 

[MyBronco15]

It's a kit by mbm brakes. It's a 1 1/8 bore. It should work for either drum, disc, or both

 

[gr8scott]

This one?
http://mbmbrakes.com/pbuft12504a-pb...co-power-brake-conversion-kit-firewall-mount/

Maybe the output rod between the booster and master needs to be adjusted just a little bit out?

My son used to love helping me work on the Bronco

 

[MyBronco15]

Yes that's it. Distributed by pirate jack which is their on distributor. Pirate jack said 1 1/8 that says 1. It does list it as disc/drum but Im not ready to tear the front axle apart yet. I've got a dana44 from 79 truck sitting in my garage for it but not sure when I'll be swapping parts. I wouldn't think it would cause that much play. I'll reblead this weekend maybe and see if any improvement. I'll adjust the pushrod some but that's only the first half inch. Not sure where the other two inches is going though.

Yea, the boy loves it. But we all enjoy riding in it more. Almost highway ready but not until I get a rollbar in there..... ��

 

[DirtDonk]

On any master/booster installation you need to verify that the booster rod is almost contacting the back of the master piston when in it's static position. The spec is something like 1/64" or less, but someone posted up a stat from a Ford book not long ago that we might be able to find.
With a pedal ratio of (for example) 6:1 just a 1/4" gap in the rod to the back of the piston is going to net you 1 1/2" of pedal free-travel. Not good.
So you see how it has to be a very tight gap to net you a firm pedal. You can't have them touch by more than a tiny bit however, or you have all sorts of other issues.
The best bet is to keep a tiny, tiny gap.
Sorry I don't know the exact spec.

Maybe it was on one of Steve83's SuperMotors pages it's documented?
Anyway, that's probably the most common reason for excessive pedal travel when changing a master, booster, or both.

Paul