master cylinder ? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

boilrman · 12-13-2002, 10:28 AM

I have had a spongy brake pedal for a few days and finally stopped by the parts store and they said I have a leak somewhere.

I took the lid of the master cylinder and the fluid from the front compartment started to flow out. It was full and under pressure. The rear compartment had about 1/2" of fluid in it. I refilled the rear compartment and the brake pedal has returned to what it was before.

My ? is, why was the front compartment pressurized?

I haven't pulled the drums yet to look for leaks, but I dont understand why the front compartment was under pressure.

Any help would be appreciated.

Thanks

Chad

Longhorn Man · 12-13-2002, 01:10 PM

Very interesting...
If it was pressurized, you'd think it would cause the brakes to drag.
No answer here...just a TTT.

da-burb · 12-13-2002, 02:20 PM

My best guess is the seal between the 2 reservoirs has failed. You are pumping the fluid from the back into the front. you will need a new master cylinder.

boilrman · 12-13-2002, 02:51 PM

Da Burb,

My thought also was that the master cylinder was failing, but the parts store folks seem to think there is a leak somewhere.

But also I havent noticed any fluid leaking from the master cylinder before, and when I took the cover off the back half of it was almost empty. I dont see how it could be going empty unless there was a leak somewhere, but yet I am not sure how I am pressurizing the front half, unless of course like you mentioned the seals in the master cylinder bad forcing it into the front.

Unfortunately I dont have the time now to look into it for a few days, so I will just have to keep an eye on it and see how things go.

Any other ideas would be appreciated.

da-burb · 12-13-2002, 08:58 PM

The other leak that you sometimes can't see is when the fluid is draining into your vacuum booster. All I can say is,where did the extra fluid come from in the front reservoir? If there is more there than there should be, it had to come from the back reservoir. (Unless you recently put in new pads/shoes and never checked the master cylinder fluid level)

Longhorn Man · 12-14-2002, 12:22 AM

And if there is no booster, it'll leak into the cab down under the carpet.

Capitol68 · 12-14-2002, 01:45 AM

are your brake shoes so far worn that it has used that much fluid to function,,dont know about pressure question ?

boilrman · 12-14-2002, 03:02 AM

truck has new pads on all four corners. No power brakes. was just doing some minor metal work on drivers side floor and saw no leaks(I had the rubber pulled all the way over to the gas pedal)

It is my '70 3/4t, so drum brakes all around. I replaced one front and one rear wheel cylinder that was leaking when I did the brake job.

Capitol68 · 12-14-2002, 09:50 AM

sometimes when u do a brake job and the shoes were worn bad and u added fluid previously it took up the added space. when u replaced them the extra fluid had to go somewhere, hence seemingly more fluid. I would just keep an eye on it, make sure shoes are all adjusted up.

tai · 12-14-2002, 05:12 PM

dont know about the front presure, but sounds a lot like the 30 seconds before my master cylinder went out

12-13-2002, 10:28 AM	#1
boilrman 70 Chevy 3/4t Join Date: Sep 2000 Location: Kokomo,IN USA Posts: 1,230	master cylinder ? I have had a spongy brake pedal for a few days and finally stopped by the parts store and they said I have a leak somewhere. I took the lid of the master cylinder and the fluid from the front compartment started to flow out. It was full and under pressure. The rear compartment had about 1/2" of fluid in it. I refilled the rear compartment and the brake pedal has returned to what it was before. My ? is, why was the front compartment pressurized? I haven't pulled the drums yet to look for leaks, but I dont understand why the front compartment was under pressure. Any help would be appreciated. Thanks Chad __________________ Find a job you love and you'll never have to work a day in your life. 1998 Yamaha V-Star 650 1970 Chevy 3/4t, I6 (rebuilt), 4 speed Kokomo,IN

12-13-2002, 02:20 PM	#3
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	My best guess is the seal between the 2 reservoirs has failed. You are pumping the fluid from the back into the front. you will need a new master cylinder. __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087

12-13-2002, 02:51 PM	#4
boilrman 70 Chevy 3/4t Join Date: Sep 2000 Location: Kokomo,IN USA Posts: 1,230	Da Burb, My thought also was that the master cylinder was failing, but the parts store folks seem to think there is a leak somewhere. But also I havent noticed any fluid leaking from the master cylinder before, and when I took the cover off the back half of it was almost empty. I dont see how it could be going empty unless there was a leak somewhere, but yet I am not sure how I am pressurizing the front half, unless of course like you mentioned the seals in the master cylinder bad forcing it into the front. Unfortunately I dont have the time now to look into it for a few days, so I will just have to keep an eye on it and see how things go. Any other ideas would be appreciated. __________________ Find a job you love and you'll never have to work a day in your life. 1998 Yamaha V-Star 650 1970 Chevy 3/4t, I6 (rebuilt), 4 speed Kokomo,IN

12-13-2002, 08:58 PM	#5
da-burb "Ochre Ogre" Join Date: Apr 2000 Location: Springfield, MN Posts: 3,558	The other leak that you sometimes can't see is when the fluid is draining into your vacuum booster. All I can say is,where did the extra fluid come from in the front reservoir? If there is more there than there should be, it had to come from the back reservoir. (Unless you recently put in new pads/shoes and never checked the master cylinder fluid level) __________________ Bowtie Truck Stop Inc. Mid-West GM Truck Restoration Parts Supplier Your Key Parts, Auto Metal Direct, Dynacorn, and Goodmark dealer. like us @ www.facebook.com/BowtieTruckStop 1971 C-10 Suburban (Ochre) 1971 K-10 Suburban (Ochre) 1972 C-10 Suburban (Ochre) 1972 K-20 Suburban (Yellow- that just aint right!) Springfield, Minnesota 56087

12-14-2002, 01:45 AM	#7
Capitol68 Registered User Join Date: Sep 2002 Location: Austin,Tx Posts: 327	are your brake shoes so far worn that it has used that much fluid to function,,dont know about pressure question ? __________________ '68 Fleet LWB orig 307 .030 over GM Hei/w Accel Super coil orig. stock TH 400. _______________________ “What we do for ourselves dies with us. What we do for others and the world remains and is immortal.” Albert Pine English author d.1851

12-13-2002, 01:10 PM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	Very interesting... If it was pressurized, you'd think it would cause the brakes to drag. No answer here...just a TTT.

12-14-2002, 12:22 AM	#6
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	And if there is no booster, it'll leak into the cab down under the carpet.

12-14-2002, 03:02 AM	#8
boilrman 70 Chevy 3/4t Join Date: Sep 2000 Location: Kokomo,IN USA Posts: 1,230	truck has new pads on all four corners. No power brakes. was just doing some minor metal work on drivers side floor and saw no leaks(I had the rubber pulled all the way over to the gas pedal) It is my '70 3/4t, so drum brakes all around. I replaced one front and one rear wheel cylinder that was leaking when I did the brake job. __________________ Find a job you love and you'll never have to work a day in your life. 1998 Yamaha V-Star 650 1970 Chevy 3/4t, I6 (rebuilt), 4 speed Kokomo,IN

12-14-2002, 09:50 AM	#9
Capitol68 Registered User Join Date: Sep 2002 Location: Austin,Tx Posts: 327	sometimes when u do a brake job and the shoes were worn bad and u added fluid previously it took up the added space. when u replaced them the extra fluid had to go somewhere, hence seemingly more fluid. I would just keep an eye on it, make sure shoes are all adjusted up. __________________ '68 Fleet LWB orig 307 .030 over GM Hei/w Accel Super coil orig. stock TH 400. _______________________ “What we do for ourselves dies with us. What we do for others and the world remains and is immortal.” Albert Pine English author d.1851

12-14-2002, 05:12 PM	#10
tai Registered User Join Date: Dec 2002 Location: bend oregon Posts: 32	tai71k20 dont know about the front presure, but sounds a lot like the 30 seconds before my master cylinder went out



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