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

Hotcorner · 07-27-2018, 08:39 AM

So the rear half of the master cylinder slowly loses fluid until empty. I suspected that the front wheel cylinders might be leaking, but I pulled back the rubber and they were dry inside. I see no evidence of leakage inside the drum. I have looked and can’t seem to find a leaking line anywhere.

It’s a 69 K25 with PB and drums on all 4 corners.

I’m stumped. Ideas? Pump the fire out of the pedal and keep looking, I guess.

volksworld · 07-27-2018, 08:51 AM

its leaking out of the rear seal of the master into the brake booster

Hotcorner · 07-27-2018, 08:57 AM

Quote:

Originally Posted by volksworld

its leaking out of the rear seal of the master into the brake booster

Thanks, brother!

**Rickysnickers** · 07-27-2018, 09:15 AM

Did you also check your rear wheel cylinders? I'm assuming you see no leaks under the truck either. If not, then I suspect what Volksworld said is probably the case. Disconnect the master cylinder from the power booster and see if you see any fluid there.

BLT gmc · 07-27-2018, 09:33 AM

All good advise. I have a truck that would suck the fluid through the vacuum hose and burn the brake fluid, with very little evidence. There was an internal hole/defect in the seal/bladder in the vacuum canister of the power brakes. Just an idea if all other sources check out okay, Bruce

Hotcorner · 07-27-2018, 09:39 AM

Rear cylinders are good. Replaced a couple months ago. No leaks, other than motor and t/c.

cebra · 07-27-2018, 11:01 AM

Quote:

Originally Posted by volksworld

its leaking out of the rear seal of the master into the brake booster

X2, I couldn't find mine either as I never looked up underneath there. New master which was surprisingly cheap and easy to install, no problems since.

Hotcorner · 07-27-2018, 01:01 PM

Quote:

Originally Posted by volksworld

its leaking out of the rear seal of the master into the brake booster

Correct, Sir! Ding ding ding? $60 for a new one. $30 for reman.

68Gold/white · 07-27-2018, 02:30 PM

Quote:

Originally Posted by Hotcorner

Correct, Sir! Ding ding ding? $60 for a new one. $30 for reman.

Do NOT EVER buy a reman master cyl!!!

Hotcorner · 07-27-2018, 05:23 PM

Quote:

Originally Posted by 68Gold/white

Do NOT EVER buy a reman master cyl!!!

I know. I don’t buy much reman because I prefer to only fix a problem one time. Had already gotten a brand new one.

07-27-2018, 08:39 AM	#1
Hotcorner Registered User Join Date: Apr 2017 Location: Knoxville, Tennessee Posts: 890	Vanishing Brake Fluid So the rear half of the master cylinder slowly loses fluid until empty. I suspected that the front wheel cylinders might be leaking, but I pulled back the rubber and they were dry inside. I see no evidence of leakage inside the drum. I have looked and can’t seem to find a leaking line anywhere. It’s a 69 K25 with PB and drums on all 4 corners. I’m stumped. Ideas? Pump the fire out of the pedal and keep looking, I guess. __________________ 1969 GMC K25 2007 Chevy 2500HD Classic Duramax 1999 Tahoe 2DR 4WD Base Model 5.7l

07-27-2018, 08:51 AM	#2
volksworld Registered User Join Date: Apr 2011 Location: formerly NY currentlyNC Posts: 389	Re: Vanishing Brake Fluid its leaking out of the rear seal of the master into the brake booster __________________ 1970 short step c10 leaf spring 454

07-27-2018, 09:15 AM	#4
Rickysnickers Senior Member Join Date: Oct 2014 Location: Eagle, ID Posts: 2,969	Re: Vanishing Brake Fluid Did you also check your rear wheel cylinders? I'm assuming you see no leaks under the truck either. If not, then I suspect what Volksworld said is probably the case. Disconnect the master cylinder from the power booster and see if you see any fluid there.

07-27-2018, 09:33 AM	#5
BLT gmc Registered User Join Date: Feb 2014 Location: West Iowa Posts: 1,106	Re: Vanishing Brake Fluid All good advise. I have a truck that would suck the fluid through the vacuum hose and burn the brake fluid, with very little evidence. There was an internal hole/defect in the seal/bladder in the vacuum canister of the power brakes. Just an idea if all other sources check out okay, Bruce

07-27-2018, 09:39 AM	#6
Hotcorner Registered User Join Date: Apr 2017 Location: Knoxville, Tennessee Posts: 890	Re: Vanishing Brake Fluid Rear cylinders are good. Replaced a couple months ago. No leaks, other than motor and t/c. __________________ 1969 GMC K25 2007 Chevy 2500HD Classic Duramax 1999 Tahoe 2DR 4WD Base Model 5.7l



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