Braking Problem. Do I need a Vacuum Canister? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

AusTx68 · 02-17-2012, 03:01 PM

Since adding rear disc brakes my braking has suffered. When cruising down the road, the first time I hit the brakes they all grab really well. Each push of the pedal immediately after results in harder pedal with less braking action. If I need to quickly slow down after the initial pedal I have to really get on it. It can be a little scary at times. I'm guessing it's a vacuum issue but I'm not sure. Here is my setup:

New Stock 11" Vacuum Booster
71/72 Front Disc Brakes
Captainfab Rear Brake Setup (Camaro calipers)
CPP Master Cylinder MCPV-1 (has built-in prop valve)

The master cylinder is adjustable. I have pressure to the rear brakes set to max.
My cam duration 245/245 @ .050" lift

68gmsee · 02-17-2012, 03:15 PM

It may be a proportional valve or adjustment problem

The front brakes are supposed to do the majority of the braking in a vehicle. Some where around 60:40 or 70:30 not exact on the numbers. If the rear are doing the majority of the braking it will be a lot harder to stop.

Don't overlook the possibility of collapsed rubber lines if they haven't been replaced.

forgot to include. It is possible the booster is at fault if it's losing it's vacuum but check the easiest and cheapest first.

cdowns · 02-17-2012, 07:54 PM

when adding disc to rear did you change to the proper master cyl and proportioning valves

AusTx68 · 02-17-2012, 10:01 PM

The master cylinder has a built-in prop valve and works with a disc/disc setup. My brake lines are stainless or steel braided with the exception of the short lines to the rear calipers. I will mess with the adjustment on the master cylinder and see what happens. I hate that my disc/disc braking is worse than my original disc/drum.
Posted via Mobile Device

69Beauty · 02-17-2012, 10:52 PM

Have you done a vacuum test to see just how much it pulls at idle? thats what i would do if i had a cam that big in it.

RichardJ · 02-17-2012, 11:01 PM

Depress the pedal several times and then hold the pedal down. Start the engine. The pedal should go down as you are pushing with the same pressure. If the pedal does not move you could have a vacuum leak before the booster or the booster could be bad.

Second test.
Depress the brake pedal with the engine running. Turn the engine off. The pedal should stay the same. If the pedal moves up the booster is bad.

02-17-2012, 03:01 PM	#1
AusTx68 Registered User Join Date: Nov 2008 Location: Italy Posts: 1,278	Braking Problem. Do I need a Vacuum Canister? Since adding rear disc brakes my braking has suffered. When cruising down the road, the first time I hit the brakes they all grab really well. Each push of the pedal immediately after results in harder pedal with less braking action. If I need to quickly slow down after the initial pedal I have to really get on it. It can be a little scary at times. I'm guessing it's a vacuum issue but I'm not sure. Here is my setup: New Stock 11" Vacuum Booster 71/72 Front Disc Brakes Captainfab Rear Brake Setup (Camaro calipers) CPP Master Cylinder MCPV-1 (has built-in prop valve) The master cylinder is adjustable. I have pressure to the rear brakes set to max. My cam duration 245/245 @ .050" lift __________________ '68 C-10 SWB 383 Stroker (Sold 3/2/2013) '87 R2500 LWB 454 TBI converted to Carb

02-17-2012, 03:15 PM	#2
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: Braking Problem. Do I need a Vacuum Canister? It may be a proportional valve or adjustment problem The front brakes are supposed to do the majority of the braking in a vehicle. Some where around 60:40 or 70:30 not exact on the numbers. If the rear are doing the majority of the braking it will be a lot harder to stop. Don't overlook the possibility of collapsed rubber lines if they haven't been replaced. forgot to include. It is possible the booster is at fault if it's losing it's vacuum but check the easiest and cheapest first. Last edited by 68gmsee; 02-17-2012 at 03:17 PM. Reason: add info.

02-17-2012, 07:54 PM	#3
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: Braking Problem. Do I need a Vacuum Canister? when adding disc to rear did you change to the proper master cyl and proportioning valves __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

02-17-2012, 10:01 PM	#4
AusTx68 Registered User Join Date: Nov 2008 Location: Italy Posts: 1,278	Re: Braking Problem. Do I need a Vacuum Canister? The master cylinder has a built-in prop valve and works with a disc/disc setup. My brake lines are stainless or steel braided with the exception of the short lines to the rear calipers. I will mess with the adjustment on the master cylinder and see what happens. I hate that my disc/disc braking is worse than my original disc/drum. Posted via Mobile Device __________________ '68 C-10 SWB 383 Stroker (Sold 3/2/2013) '87 R2500 LWB 454 TBI converted to Carb

02-17-2012, 10:52 PM	#5
69Beauty Registered User Join Date: Aug 2010 Location: North Dakota Posts: 279	Re: Braking Problem. Do I need a Vacuum Canister? Have you done a vacuum test to see just how much it pulls at idle? thats what i would do if i had a cam that big in it. __________________ 1969 C-10 LWB 388 SBC Vortec Heads 10:1 Lunati 60103 cam Weiand intake Quadrajet Carb 12 Bolt 3:73 posi 700r4 Hedman Headers 1998 Dodge Ram 1500 Reg. Cab Long Box 4X4 5.9L 183k miles 1956 Ford F-100 Longbed with Original 272 and 3OTT 1995 C1500 ECSB 6.5 Diesel 4l80e (Camper puller) 1997 Dodge 3/4 ECLB V10 (MPG machine)

02-17-2012, 11:01 PM	#6
RichardJ Registered User Join Date: Apr 2010 Location: So Cal Posts: 1,423	Re: Braking Problem. Do I need a Vacuum Canister? Depress the pedal several times and then hold the pedal down. Start the engine. The pedal should go down as you are pushing with the same pressure. If the pedal does not move you could have a vacuum leak before the booster or the booster could be bad. Second test. Depress the brake pedal with the engine running. Turn the engine off. The pedal should stay the same. If the pedal moves up the booster is bad. __________________ '67 GMC 2500, 292, 4spd, AC



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