Hard brake pedal

[KF100TK]

I am having a hard time with this.
I got the power disc conversion kit from MP brakes comes with everything new, got it all installed had a hard time bleeding it
but resolved with another new MC. Everything worked except it did'nt feel like the power booster was working .
I left it alone for 8 months then read in classic trucks dec/08 issue (they did this kit on a 68) that they had to widen the holes on
the firewall brackets, so i figured that was my problem with the bracket was pinching the swing arm to tight.
I adjusted everything now the brake pedal feels like a rock and does'nt even move, what gives? I separated the MC from the booster
and tried the pedal and works great, started the truck and the booster made the pedal even better but soon as i put the MC back
on the pedal wouldn't budge. I tried cracking the bleeders pedal was very hard to move but did, closed the bleeders no difference.

[FORDification]

Well, sounds like you've isolated the problem to the master cylinder. What MC did you replace with? You might want to call MP tech support with your problem and make sure that whatever MC you have now will work with their setup. Did you experience any problems when you installed that MC the second time?

[KF100TK]

Thanks for the reply Keith, i did call MP brakes they told me to test the booster first by placing a block across the booster
and start the truck, press the pedal and listen for a air leak. The booster did not leak. The MC was the stlye they supplied
me with (corvette disc/disc set up with new proportioning valve), the first one failed ,2nd leaked, 3rd was ok until now.
The 3rd MC worked prior to me separating the MC from the booster other than it felt i still had manual brakes.
I left a message with MP brakes but am still waiting a reply. Hoping you guys could help as well.

[Wes]

I wish I knew why all these after market kit makers use the disc disc vette mc. I don't see the reason for it, my buddies model A pu has a S10 rear drum axle, 80 something camero front discs the vette disc disc mc. He had a problem with the rears not working, after all was said and done the master cylinder was to blame. I would attempt to find a different application mc to use.

[fordman]

disc disc? did you switch the rear to discs also? if not you need a disc drum master. also it sounds like there is a clog in the brake fluid system some place. making the fluid not want to flow through the lines and to the wheels. you can take off one line at a tiem on each whell and push the brakes and see where the clog is at.

[Rusty67]

disc disc? did you switch the rear to discs also? if not you need a disc drum master. also it sounds like there is a clog in the brake fluid system some place. making the fluid not want to flow through the lines and to the wheels. you can take off one line at a tiem on each whell and push the brakes and see where the clog is at.

The only difference (that I know of) between a disc and drum MC is that drum MCs have residual pressure valves built in. If there is no residual pressure valve, its a disc MC. You can put a residual pressure valve in and drum circuit to accomplish the same thing that swaping the MC would with much less time/effort.

Of course, there are other things to consider. What size bore is the MC and what size is the piston bore in the calipers ? Check for stuck wheel cylinders/calipers. If you have stuck pistons or wheel cylinders, your pedal will be rock hard.

Also, check your proportioning valve. If it is one of those that has a slide pin in it then that could easily be causing the problem, it may be shuting off a circuit which lost pressure. This can cause a hard pedal and those dumb things do brake (ask me how I know....). Crack both the brake lines that come from the MC and screw into the proportioning valve and see if the pedal pressure changes.

Make sure you bench bleed the MC before you put it in.