Leaky master cylinder (72) - The 1947 - Present Chevrolet & GMC Truck Message Board Network

jtminton · 03-22-2005, 05:28 PM

I am having a problem with a leaky master cylinder. I noticed that the fluid was leaking out of the front of the cylinder a few days ago. When I opened the lid, more fluid poured out of the front. The back chamber in the reservoir was about 2/3 full. I thought it might be the gasket, so I refilled it and put on a new lid and gasket that I had laying around. It's still doing it, although less than before.

The truck has been recently put back on the road after 6 years of pasture time. I don't know if that makes any difference in what is happening. Any ideas?

franko72 · 03-22-2005, 08:48 PM

look at the mating surface of the master cylinder where the gasket seats. take your finger and feel for rough spots from possible corrosion or rust. If so, take some fine emery paper and clean the surface. after cleaning wipe surface with a little brake fluid on a rag.That has happened to me before on one of my rides that sat for a while, I cleaned the surface and it stopped . Good luck.

**PICKMUP** · 03-23-2005, 12:35 AM

If that truck has been sitting outside for six years, go get a rebuilt MS. I wouldn't trust it, anyway.

Zonaman · 03-23-2005, 11:15 AM

If it's leaking from the lid you can take a length of thin nylon chord, coat it in disc brake anti-squeel (that orange stuff),fix it to the lid, and let it dry It works as an extra gasket. Should stop that source of leakage.

jtminton · 03-23-2005, 11:53 AM

I have a never-been-used master cylinder from a 73. That's where I got the new cap. I just haven't been able to muster up the energy to mess with it yet, but I'm sure I will get to it.

72K10 · 03-23-2005, 08:39 PM

The seal between the rear and front chambers is shot, common in vehicles that have been sitting for a long time. This will cause the fluid from the rear to get pushed into the front, which will then overflow, as it's doing with yours. Easiest fix is to get a new master cylinder. Cost is as little as $12 at Autozone, and it takes only a half hour or so to bench bleed and install. Hope this helps.

cheyenne10 · 03-24-2005, 01:58 AM

72K10 has it on the head. Your loosing fluid past the seal to the front reservoir. Often goes with the low brake pedal in my experience. If I remember rear reservoir is for front brakes aka most of your stopping power.

had a few scares before. Also had some condition with "new" rebuilt unit. I got to do swap twice. Well worth the time. (yours and my lives)

72K10 · 03-24-2005, 09:13 AM

I wasn't too clear; it's not the seal on the top. It's internal. Just so you don't try to seal it across the top; won't work.

jtminton · 03-24-2005, 10:23 AM

I guess I should get around to replacing it a little sooner than I planned to. Thanks 72K10.

jtminton · 03-29-2005, 10:52 AM

I still haven't replaced the MC, so I was doing my usual check to see if I needed to add any brake fluid. When I opened the hood, I could hear the MC sort of hissing and bubbling like it was releasing pressure. My question is: should there be pressure in the MC when the truck is not running? I had just gone on a short little trip (about 2 miles) and there was about 5 minutes between the time I shut off the engine and opened the hood.

Just didn't seem normal.

cheyenne10 · 03-29-2005, 04:14 PM

I have never heard of pressure in master cyl. when it is at rest. When you are pushing on pedal fluid with push out though. You probably know how corrrosive regualr brake fluid is. Keep it washed out of engine compartment.

stickshift_or_walk · 03-29-2005, 07:08 PM

The only reason I can imagine for pressure buildup in the MC would be excessive heat in one of the wheel cylinder/calipers. That would indicate something needing immediate attention.

After your next drive, check all of your drums/rotors/backing plates/calipers for heat.

SSOW

jtminton · 04-05-2005, 10:46 AM

Quote:

Originally Posted by stickshift_or_walk

The only reason I can imagine for pressure buildup in the MC would be excessive heat in one of the wheel cylinder/calipers. That would indicate something needing immediate attention.

After your next drive, check all of your drums/rotors/backing plates/calipers for heat.

SSOW

Thanks for the tip. After my drive home, I felt all the wheels and the right rear was REALLY hot and I could smell brakes. I had put on the parking brake a few days back when I was working in the driveway. Since it is very rarely used and since the truck spent the last 6 years in a pasture, the cables and sheath are pretty rusty. I pounded on the sheaths with a hammer on both sides and it seemed to fix the problem. Lots of rusty dust came out.

Anybody have a cheap cable in good shape for a 72 3/4 LWB with a HO52 rear end?

03-22-2005, 05:28 PM	#1
jtminton II Timothy 4:2 Join Date: Mar 2005 Location: Fort Worth Posts: 361	Leaky master cylinder (72) I am having a problem with a leaky master cylinder. I noticed that the fluid was leaking out of the front of the cylinder a few days ago. When I opened the lid, more fluid poured out of the front. The back chamber in the reservoir was about 2/3 full. I thought it might be the gasket, so I refilled it and put on a new lid and gasket that I had laying around. It's still doing it, although less than before. The truck has been recently put back on the road after 6 years of pasture time. I don't know if that makes any difference in what is happening. Any ideas? __________________ 72 C-20 2WD LWB Custom Camper Deluxe 350 4 Speed "Project Lazarus" MY TRUE PRIDE AND JOY "Dangit, Bobby, people will see that oil stain and think I'm a drunk."

03-22-2005, 08:48 PM	#2
franko72 Franko72 Join Date: May 2004 Location: Cincinnati,Ohio Posts: 471	look at the mating surface of the master cylinder where the gasket seats. take your finger and feel for rough spots from possible corrosion or rust. If so, take some fine emery paper and clean the surface. after cleaning wipe surface with a little brake fluid on a rag.That has happened to me before on one of my rides that sat for a while, I cleaned the surface and it stopped . Good luck. __________________ 1972 C20 cheyenne Super 402 BB/400 turbo LWB A/C Tilt, Cruise Control, Saddle tanks, Tach My Home Page Better to burn out than it is to rust

03-23-2005, 12:35 AM	#3
PICKMUP "Trucks with Class" Join Date: Jul 2000 Location: OLYMPIA, WA, USA Posts: 8,134	If that truck has been sitting outside for six years, go get a rebuilt MS. I wouldn't trust it, anyway. __________________ Email: frankeham@comcast.net Phone: 360-956-7170

03-23-2005, 11:15 AM	#4
Zonaman Registered User Join Date: May 2004 Location: Chandler, AZ Posts: 519	If it's leaking from the lid you can take a length of thin nylon chord, coat it in disc brake anti-squeel (that orange stuff),fix it to the lid, and let it dry It works as an extra gasket. Should stop that source of leakage. __________________ 69 C-10 LWB, 350, 700r4, 3.73 posi-traction, front disc brakes. Built for the desert- She can take the heat! Vivat Rex Chevus!

03-23-2005, 11:53 AM	#5
jtminton II Timothy 4:2 Join Date: Mar 2005 Location: Fort Worth Posts: 361	I have a never-been-used master cylinder from a 73. That's where I got the new cap. I just haven't been able to muster up the energy to mess with it yet, but I'm sure I will get to it. __________________ 72 C-20 2WD LWB Custom Camper Deluxe 350 4 Speed "Project Lazarus" MY TRUE PRIDE AND JOY "Dangit, Bobby, people will see that oil stain and think I'm a drunk."

03-23-2005, 08:39 PM	#6
72K10 Registered User Join Date: Dec 1999 Location: New York Posts: 288	The seal between the rear and front chambers is shot, common in vehicles that have been sitting for a long time. This will cause the fluid from the rear to get pushed into the front, which will then overflow, as it's doing with yours. Easiest fix is to get a new master cylinder. Cost is as little as $12 at Autozone, and it takes only a half hour or so to bench bleed and install. Hope this helps. __________________ RLTW 70C10 72K10 72K10 Cheyenne 72C10 Cheyenne Super

03-24-2005, 01:58 AM	#7
cheyenne10 Senior Member Join Date: Apr 2000 Location: Oregon Posts: 2,512	72k10 72K10 has it on the head. Your loosing fluid past the seal to the front reservoir. Often goes with the low brake pedal in my experience. If I remember rear reservoir is for front brakes aka most of your stopping power. had a few scares before. Also had some condition with "new" rebuilt unit. I got to do swap twice. Well worth the time. (yours and my lives) __________________ Bob

03-24-2005, 09:13 AM	#8
72K10 Registered User Join Date: Dec 1999 Location: New York Posts: 288	I wasn't too clear; it's not the seal on the top. It's internal. Just so you don't try to seal it across the top; won't work. __________________ RLTW 70C10 72K10 72K10 Cheyenne 72C10 Cheyenne Super

03-24-2005, 10:23 AM	#9
jtminton II Timothy 4:2 Join Date: Mar 2005 Location: Fort Worth Posts: 361	I guess I should get around to replacing it a little sooner than I planned to. Thanks 72K10. __________________ 72 C-20 2WD LWB Custom Camper Deluxe 350 4 Speed "Project Lazarus" MY TRUE PRIDE AND JOY "Dangit, Bobby, people will see that oil stain and think I'm a drunk."

03-29-2005, 10:52 AM	#10
jtminton II Timothy 4:2 Join Date: Mar 2005 Location: Fort Worth Posts: 361	Update question I still haven't replaced the MC, so I was doing my usual check to see if I needed to add any brake fluid. When I opened the hood, I could hear the MC sort of hissing and bubbling like it was releasing pressure. My question is: should there be pressure in the MC when the truck is not running? I had just gone on a short little trip (about 2 miles) and there was about 5 minutes between the time I shut off the engine and opened the hood. Just didn't seem normal. __________________ 72 C-20 2WD LWB Custom Camper Deluxe 350 4 Speed "Project Lazarus" MY TRUE PRIDE AND JOY "Dangit, Bobby, people will see that oil stain and think I'm a drunk."

03-29-2005, 04:14 PM	#11
cheyenne10 Senior Member Join Date: Apr 2000 Location: Oregon Posts: 2,512	TTT for you I have never heard of pressure in master cyl. when it is at rest. When you are pushing on pedal fluid with push out though. You probably know how corrrosive regualr brake fluid is. Keep it washed out of engine compartment. __________________ Bob

03-29-2005, 07:08 PM	#12
stickshift_or_walk Left Foot Functioning Join Date: May 2004 Location: Duncan, OK Posts: 348	The only reason I can imagine for pressure buildup in the MC would be excessive heat in one of the wheel cylinder/calipers. That would indicate something needing immediate attention. After your next drive, check all of your drums/rotors/backing plates/calipers for heat. SSOW __________________ 69 C10 (#1 son's ride) 72 C20 (Dad's farm truck) 82 K30 (Ex - brush pumper project)



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