Brake puzzler - The 1947 - Present Chevrolet & GMC Truck Message Board Network

krue · 08-24-2005, 10:30 PM

My brake pedal gets harder (less play) to mash the more I drive my truck until it gets to the point that I have to unbolt the M/c from the booster and bolt it back up. Then I have a few inches of play in my brakes which goes away in a vicious circle. Background: Power drums, happened with 2 different boosters, 2 different m/c's, the wheel cylinders were replaced about a year ago, and the pass side rubber hose was replaced at the same time as the wheel cylinders.

*FormerMember* · 08-24-2005, 10:53 PM

Got a disk proportion valve in place of a drum one or vice versa?

Push rod too long?

frogsmakr · 08-24-2005, 10:58 PM

get the brake line off the header.

matt67350 · 08-24-2005, 11:00 PM

look in this info.

http://www.thebrakeman.com/valvetechi

We may as well start with the residual valve, because it is the first one that should be determined whether or not it is needed. This valve does exactly as its name suggests. It keeps a pre-determined amount of residual pressure in the line after you remove your foot from the brake pedal. This aids in preventing excessive pedal travel as well as insuring consistent height to the pedal. In a drum brake, heavy return springs are present to pull the shoes away from the drums. When not in use, the shoes are pulled back until they rest on a centering pin, usually located at the 12:00, or top position, on the backing plate also holding the wheel cylinder. In order to avoid the excessive pedal travel to move enough fluid from the master to activate the shoes, a 10-12 pound residual valve is installed in the line. Sine the return springs are stronger than the 12 pound valve, the shoes are pulled away from the drum in spite of the resistance so no brake drag results.

good luck,
Matthew

*FormerMember* · 08-24-2005, 11:02 PM

This is why I was asking about his proportioning valve.

In GM's, the PV also had the RV in it.

I had a jeep that did this, when i swapped to rear disk brakes.

krue · 08-24-2005, 11:11 PM

Where is the residual valve?

*FormerMember* · 08-24-2005, 11:16 PM

Quote:

Originally Posted by krue

Where is the residual valve?

In a Gm, it's in the back half of the proportioning valve.

jhow66 · 08-24-2005, 11:26 PM

Rod between booster and master cylinder does not have enough free play. Check it by taking the MC loose from booster-start engine and check rod length from booster surface and then check distance from MC mounting surface to inside of MC.

stllookn · 08-25-2005, 02:30 AM

Had this happen on my son's '77 truck when we installed power assist. The actuating rod was not adjustable and was just a tad too long. After about 20 - 30 minutes of driving the brakes were dragging and we had to stop and let them cool. Had to shim the MC from the fire wall with a set of washers to get the right amount of clearance. Worked great after that.

Gordo · 08-25-2005, 08:31 PM

I'm with jhow66 and stllookn. The actuating rod needs free play if only a 1/16th of an inch. If the M/C piston is not allowed to return 100% you will have those syptoms.

**pjmoreland** · 08-26-2005, 01:10 AM

I had that happen once when I adjusted the rod that connects to the brake pedal so that the brake pedal bottomed out on the little rubber bumper under the dash. I'm guessing that pressure was being trapped inside the master cylinder. Is there any chance that the big tension coil spring that pulls your brake pedal back up is broken?

3 on the tree · 08-26-2005, 09:08 AM

I vote pushrod being too long. I had the same thing happen to me when I converted a 70 F350 to power brakes. The booster and MC were not a matched set, and it took me 3 tries to get the correct length pushrod. Nothing like heading the hiway and brakes engage while you are in the fast lane!!!

08-24-2005, 10:30 PM	#1
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Brake puzzler My brake pedal gets harder (less play) to mash the more I drive my truck until it gets to the point that I have to unbolt the M/c from the booster and bolt it back up. Then I have a few inches of play in my brakes which goes away in a vicious circle. Background: Power drums, happened with 2 different boosters, 2 different m/c's, the wheel cylinders were replaced about a year ago, and the pass side rubber hose was replaced at the same time as the wheel cylinders. __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

08-24-2005, 10:53 PM	#2
FormerMember Account Suspended Join Date: Feb 2004 Posts: 3,051	Got a disk proportion valve in place of a drum one or vice versa? Push rod too long? Last edited by FormerMember; 08-24-2005 at 11:03 PM.

08-24-2005, 11:00 PM	#4
matt67350 Registered User Join Date: May 2001 Location: Benbrook, TX Posts: 1,060	check it out look in this info. http://www.thebrakeman.com/valvetechi We may as well start with the residual valve, because it is the first one that should be determined whether or not it is needed. This valve does exactly as its name suggests. It keeps a pre-determined amount of residual pressure in the line after you remove your foot from the brake pedal. This aids in preventing excessive pedal travel as well as insuring consistent height to the pedal. In a drum brake, heavy return springs are present to pull the shoes away from the drums. When not in use, the shoes are pulled back until they rest on a centering pin, usually located at the 12:00, or top position, on the backing plate also holding the wheel cylinder. In order to avoid the excessive pedal travel to move enough fluid from the master to activate the shoes, a 10-12 pound residual valve is installed in the line. Sine the return springs are stronger than the 12 pound valve, the shoes are pulled away from the drum in spite of the resistance so no brake drag results. good luck, Matthew __________________ 69 swb stepside, rebuild and upgrade in progress

08-24-2005, 11:02 PM	#5
FormerMember Account Suspended Join Date: Feb 2004 Posts: 3,051	This is why I was asking about his proportioning valve. In GM's, the PV also had the RV in it. I had a jeep that did this, when i swapped to rear disk brakes. Last edited by FormerMember; 08-24-2005 at 11:02 PM.

08-24-2005, 11:11 PM	#6
krue Designated A-hole! Join Date: Mar 2003 Location: South Mississippi Posts: 36,450	Where is the residual valve? __________________ "If it ain't broke, fix it 'til it is!" Being stupid ain't illegal. We're Still Debt Free Except for the House!!! www.daveramsey.com 70 GMC SWB Stepside project "Green Booger" soon to be 6.0l/4l80e 93 S-10 "Poppaw's Truck"

08-24-2005, 10:58 PM	#3
frogsmakr Registered User Join Date: Feb 2000 Location: Sherwood AR Posts: 553	get the brake line off the header.

08-24-2005, 11:26 PM	#8
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Rod between booster and master cylinder does not have enough free play. Check it by taking the MC loose from booster-start engine and check rod length from booster surface and then check distance from MC mounting surface to inside of MC. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer Last edited by jhow66; 08-24-2005 at 11:27 PM.

08-25-2005, 02:30 AM	#9
stllookn Saving 1 truck at a time! Join Date: Aug 2001 Location: Kent, WA Posts: 6,465	Had this happen on my son's '77 truck when we installed power assist. The actuating rod was not adjustable and was just a tad too long. After about 20 - 30 minutes of driving the brakes were dragging and we had to stop and let them cool. Had to shim the MC from the fire wall with a set of washers to get the right amount of clearance. Worked great after that. __________________ '68 C20 Longhorn 50th Anniversary 400/TH400 '68 C20 Longhorn 50th Anniversary 468/TH400w/buckets '72 C20 Halfhorn (Longhorn w/o cab and front clip) '69 Flxible Cruiser (look up ugly in the dictionary)

08-25-2005, 08:31 PM	#10
Gordo Registered User Join Date: Jan 2001 Location: Killingworth, CT. USA Posts: 3,378	I'm with jhow66 and stllookn. The actuating rod needs free play if only a 1/16th of an inch. If the M/C piston is not allowed to return 100% you will have those syptoms. __________________ 1971 C10 swb stepside 350/700R4/3.73posi (retired as of 4/22/03) 1998 S10 short bed 2002 S10 Blazer 1942 Oldsmobile 1958 Massey Harris Pony 1951 Wife Killingworth, Connecticut May those who love us, love us, any of those who do not love us, may God turn their hearts. And if God is unable to turn their hearts, may he turn their ankles so we may know them by their limping. A man who works with his hands is a laborer; a man who works with his hands and his brain is a craftsman; but a man who works with his hands and his brain and his heart is an artist.

08-26-2005, 01:10 AM	#11
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,426	I had that happen once when I adjusted the rod that connects to the brake pedal so that the brake pedal bottomed out on the little rubber bumper under the dash. I'm guessing that pressure was being trapped inside the master cylinder. Is there any chance that the big tension coil spring that pulls your brake pedal back up is broken?

08-26-2005, 09:08 AM	#12
3 on the tree Registered User Join Date: Oct 2004 Posts: 393	I vote pushrod being too long. I had the same thing happen to me when I converted a 70 F350 to power brakes. The booster and MC were not a matched set, and it took me 3 tries to get the correct length pushrod. Nothing like heading the hiway and brakes engage while you are in the fast lane!!!



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