Brake pedal slowly goes to floor?? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

68jimmy · 07-16-2004, 11:22 AM

Hello All,

My 68 GMC 3/4t came equipped with power assisted drum brakes on all 4 corners. I've noticed that when I'm sitting at a stop light or such that the brake pedal ever so slightly is going to the floor. What causes this? A leak in the wheel cylinders or something to do with the power assist unit itself?

thanks!

ed455 · 07-16-2004, 02:11 PM

Could be:

A) Leaking wheel cylinder which will be obvious by looking under the vehicle to see if fluid is leaking out from inside the drum. It will also be noticable by a pretty severe loss of brake fluid in the master cylinder resevoir. If it turns out to be a wheel cylinder, it will require a full rear brake job as the shoes will be fluid soaked and effectively ruined.

B) A bad master cylinder that is letting fluid bypass internally. It often leaks at the back of the master cylinder. If the brake booster is wet and the paint is peeling on the bottom of the brake booster (if it's painted), then this is the culprit. If there is no sign of a leak anywhere, then the master cylinder is the culprit in this situation as well. Master cylinders are cheap at around $25-$30 for most of OUR application.

My vote is for "B" because while wheel cylinder failure is very common, a failure that would cause an obviously dropping pedal is not common. There are other things that could cause your problem as well, but these are the most likely scenarios,

68jimmy · 07-16-2004, 03:44 PM

I haven't noticed any loss of fluid in the master cylinder, nor any wet spots or runs on any of the wheels. I'll pick up a new ms and see if that cures it...

thanks!

ed455 · 07-16-2004, 03:51 PM

Don't forget to bench-bleed it.

roccon31 · 07-24-2004, 10:33 PM

try adjusting your rear brakes...if your rear drum brakes are out of adjustment, your pedal will drift down slowly after you apply the brakes.

flatblackclassic · 08-02-2004, 09:54 PM

Quote:

Originally Posted by roccon31

try adjusting your rear brakes...if your rear drum brakes are out of adjustment, your pedal will drift down slowly after you apply the brakes.

I have this problem on my 97 but have not yet got around to adjusting the rear drums. I think I'm just going to upgrade to a rear disc setup.

K Dubb · 08-05-2004, 12:26 AM

air in the break lines will make it slowy creep to the floor

68jimmy · 08-05-2004, 06:52 PM

I will be doing a rear-end swap here in the next week, so I'll have a chance to re-bleed those again...

07-16-2004, 11:22 AM	#1
68jimmy Registered User Join Date: Dec 2003 Location: albuquerque Posts: 431	Brake pedal slowly goes to floor?? Hello All, My 68 GMC 3/4t came equipped with power assisted drum brakes on all 4 corners. I've noticed that when I'm sitting at a stop light or such that the brake pedal ever so slightly is going to the floor. What causes this? A leak in the wheel cylinders or something to do with the power assist unit itself? thanks!

07-16-2004, 02:11 PM	#2
ed455 Captain Ed Join Date: Jul 2003 Location: Port Charlotte, FL Posts: 1,241	Could be: A) Leaking wheel cylinder which will be obvious by looking under the vehicle to see if fluid is leaking out from inside the drum. It will also be noticable by a pretty severe loss of brake fluid in the master cylinder resevoir. If it turns out to be a wheel cylinder, it will require a full rear brake job as the shoes will be fluid soaked and effectively ruined. B) A bad master cylinder that is letting fluid bypass internally. It often leaks at the back of the master cylinder. If the brake booster is wet and the paint is peeling on the bottom of the brake booster (if it's painted), then this is the culprit. If there is no sign of a leak anywhere, then the master cylinder is the culprit in this situation as well. Master cylinders are cheap at around $25-$30 for most of OUR application. My vote is for "B" because while wheel cylinder failure is very common, a failure that would cause an obviously dropping pedal is not common. There are other things that could cause your problem as well, but these are the most likely scenarios, Last edited by ed455; 07-16-2004 at 02:14 PM.

07-16-2004, 03:44 PM	#3
68jimmy Registered User Join Date: Dec 2003 Location: albuquerque Posts: 431	I haven't noticed any loss of fluid in the master cylinder, nor any wet spots or runs on any of the wheels. I'll pick up a new ms and see if that cures it... thanks!

07-16-2004, 03:51 PM	#4
ed455 Captain Ed Join Date: Jul 2003 Location: Port Charlotte, FL Posts: 1,241	Don't forget to bench-bleed it.

07-24-2004, 10:33 PM	#5
roccon31 Registered User Join Date: Jul 2004 Location: sellersville pa Posts: 3	try adjusting your rear brakes...if your rear drum brakes are out of adjustment, your pedal will drift down slowly after you apply the brakes.

08-05-2004, 12:26 AM	#7
K Dubb www.Dubbmotorsports.com Join Date: Jul 2004 Location: Michigan Posts: 110	air in the break lines will make it slowy creep to the floor

08-05-2004, 06:52 PM	#8
68jimmy Registered User Join Date: Dec 2003 Location: albuquerque Posts: 431	I will be doing a rear-end swap here in the next week, so I'll have a chance to re-bleed those again...



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.