Hard Pedal After Power Disc Brake Conversion
 

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.

Re: Hard Pedal After Power Disc Brake Conversion
 

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. :metal:

Re: Hard Pedal After Power Disc Brake Conversion
 

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....).

Re: Hard Pedal After Power Disc Brake Conversion
 

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

Re: Hard Pedal After Power Disc Brake Conversion
 

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

Re: Hard Pedal After Power Disc Brake Conversion
 

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

Re: Hard Pedal After Power Disc Brake Conversion
 

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!

Re: Hard Pedal After Power Disc Brake Conversion
 

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

Re: Hard Pedal After Power Disc Brake Conversion
 

Curious..... did the kit mention any changes required or recommended w/the pedal ratio or travel?

Re: Hard Pedal After Power Disc Brake Conversion
 

It said to drill a hole in the brake pedal arm 1" lower than the stock hole if my arm only had one hole. So I did.

Re: Hard Pedal After Power Disc Brake Conversion
 

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

Re: Hard Pedal After Power Disc Brake Conversion
 

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

Re: Hard Pedal After Power Disc Brake Conversion
 

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.