Hard Pedal After Power Disc Brake Conversion

[JonnyC10]

Hey Guys,

I installed a CPP Grand Slam Kit and got it all installed. But now Im experiencing a hard brake pedal and stops just like it did when I had manual drum brakes. It hard to stop and I cant lock brakes up.

I called CPP and they told me to do a vacuum test on the intake and I am getting 15" at idle.

They told me to pump the brake and start the truck and see if the pedal goes down and it did, about a 1/4". He said that was normal. Then he had me let it idle for a few minutes, turn it off and pull the check valve out of the booster and see if I hear any air hissing, which I did. So he says the booster is working as it should but I am not getting enough vacuum. That I need 18".

So he recommends a vacuum canister or pump.

Any suggestions would be greatly appreciated. I read some threads on here about similar issues and majority say that the canister and or pump didn't produce any satisfactory results.

As far as I know, it has the stock cam. Just a plain ole 283 SBC.

[nlped]

I don't have an answer for your problem, but I can offer some insight about your current standings. My '67 with a 468ci motor produces about 14" of vacuum and the power brakes work flawlessly...but at first were terrible (mucho pedal pressure)...turned out I just had a vacuum leak that only reared it's ugly head when brakes were applied, which was a PITA to track down. Probably not your issue, but with you having 15" of vacuum, that should be plenty...

Good luck, somebody on here will have the answer, fear not.

[SCOTI]

I would check for vac leaks. A stock 283 should have more vac @ idle I would think. Check everything again (check valve for the unit installed correctly etc....). But even @ 15hg, it should work @ idle. Low vac probs will be much more noteable when you stand on the brakes after hard, WOT driving situations (not just cruising) or under repeat use of the brakes w/o time between.

If everything double checks from the engine side, I wouldn't rule out the vac booster being faulty either. I've had one that passed the same 'testing' routine but would be hard as a brick when in use (because the pressure applied was different is the only thing I could guess).

I don't know what the internal failure was but the problem went away when I swapped the vac bosster for another. Possibly hit your local auto parts store for another to 'gently' try & see if the issue persists. Be cautious so you can take it back the same day & you usually won't get grief if you have a good poker face (I didn't need this after all....).

[graymond1967]

I am having similar issues with my 67. Stock booster and master cylinder with 71 disc brakes and rear drums. I have 15 " of vacuum as well. Will double check the lines too.

If the stock booster and master cylinder are working correctly, are they adequate to stop the truck?

Ray

[Tree-50]

I heard you need a master cylinder specific for disc brakes, is yours from your drum setup?

[JonnyC10]

Originally Posted by Tree-50

I heard you need a master cylinder specific for disc brakes, is yours from your drum setup?

Me? I have the one that came with the kit. Dual reservoir for disc/drum.

[Tree-50]

Originally Posted by JonnyC10

Me? I have the one that came with the kit. Dual reservoir for disc/drum.

Whalp, sorry. Thought it might have been one of those "so easy it got overlooked" situations. Anyways, subbed in case this happens to me when I do my swap!

[SCOTI]

A vacuum pump or different booster will be needed to eliminate those possibilities (not enough vac or faulty booster).

[JonnyC10]

I ordered a Comp Cams vacuum canister. I hope that helps but I seriously doubt it.

[kitsbeach]

Let us know how it works out. I'm planning my drum to disk swap in the next month or so.

[JonnyC10]

The canister didn't help much but when I swapped my distributor for the new hei one a couple days ago, I found the the gasket for the base of the distributor was cracked. Replaced it with a new one and now I'm getting 20ish hgs of vacuum and the pedal and brakes are much better. I'm happier now.