Hard Pedal After Power Disc Brake Conversion - The 1947 - Present Chevrolet & GMC Truck Message Board Network

*JonnyC10* · 09-22-2014, 07:57 PM

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** · 09-23-2014, 08:40 AM

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 · 09-23-2014, 08:54 AM

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 · 09-23-2014, 12:33 PM

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 · 09-23-2014, 11:00 PM

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

*JonnyC10* · 09-23-2014, 11:05 PM

Quote:

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 · 09-23-2014, 11:33 PM

Quote:

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 · 09-23-2014, 11:50 PM

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

*JonnyC10* · 09-24-2014, 12:48 AM

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

**kitsbeach** · 10-09-2014, 01:56 PM

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

*JonnyC10* · 10-09-2014, 01:59 PM

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.

09-22-2014, 07:57 PM	#1
JonnyC10 Account Suspended Join Date: Jul 2014 Location: NH Posts: 280	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.

09-23-2014, 08:40 AM	#2
nlped Senior Member Join Date: Jul 2009 Location: Hayden, Al. Posts: 4,148	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. __________________ Carter 1968 Chevy SWB Fleet... Walt 1963-1/2 Ford Falcon Futura...Martha

09-23-2014, 08:54 AM	#3
SCOTI Registered User Join Date: Apr 2001 Location: DALLAS,TX Posts: 22,064	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....). __________________ 67SWB-B.B.RetroRod 64SWB-Recycle 89CCDually-Driver/Tow Truck 99CCSWB Driver All Fleetsides @rattlecankustoms in IG Building a small, high rpm engine with the perfect bore, stroke and rod ratio is very impressive. It's like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar..... *Cubic inches is like Indiana Jones with a cheap pistol.*

09-23-2014, 12:33 PM	#4
graymond1967 Registered User Join Date: Oct 2011 Location: White Rock, BC Posts: 41	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 __________________ 1967 Chev C10, in the family since 1967, iron head 502, turbo 350, white with red, unrestored. 1970 Acadian SS350, 350 with muncie 4 speed, 12 bolt, best time 13.414 @ 103.72

09-23-2014, 11:00 PM	#5
Tree-50 Registered User Join Date: Jul 2014 Location: Dallas, Texas Posts: 120	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?

09-23-2014, 11:50 PM	#8
SCOTI Registered User Join Date: Apr 2001 Location: DALLAS,TX Posts: 22,064	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). __________________ 67SWB-B.B.RetroRod 64SWB-Recycle 89CCDually-Driver/Tow Truck 99CCSWB Driver All Fleetsides @rattlecankustoms in IG Building a small, high rpm engine with the perfect bore, stroke and rod ratio is very impressive. It's like a highly skilled Morrocan sword fighter with a Damascus Steel Scimitar..... *Cubic inches is like Indiana Jones with a cheap pistol.*

09-24-2014, 12:48 AM	#9
JonnyC10 Account Suspended Join Date: Jul 2014 Location: NH Posts: 280	Re: Hard Pedal After Power Disc Brake Conversion I ordered a Comp Cams vacuum canister. I hope that helps but I seriously doubt it.

10-09-2014, 01:56 PM	#10
kitsbeach Senior Member Join Date: May 2014 Location: Vancouver, BC Posts: 591	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. __________________ Kitsbeach (Ken) 1967 C10 LWB (now SWB) Fleetside 6cyl Saginaw three-on-the-tree "No Frills" 1980 C20 Long to short & static drop "Square One"

10-09-2014, 01:59 PM	#11
JonnyC10 Account Suspended Join Date: Jul 2014 Location: NH Posts: 280	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.



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