brake fluid runs from rear of master cylinder to front then out of it - The 1947 - Present Chevrolet & GMC Truck Message Board Network

pete29 · 07-16-2009, 12:30 PM

I have a 72 chevy 4x4 3/4 ton. The brake fluid runs from the rear of master cylinder to front and then runs out of it under the cap. I put a new cap gasket in. The rear compartment of the master cylinder will run dry.
What could be the problem? Brakes work good if i keep fluid in both compartments.

mclairmo · 07-16-2009, 12:44 PM

If I am understanding your problem correctly, it sounds like maybe a piston seal is leaking and you are losing fluid to the outside. You need a new master cylinder. Check under the master cylinder near the firewall (manual brakes) or power booster to see if it is wet under there. The fluid would have to leak out of the bore at the back of the master cylinder where it meets the firewall or the power booster. If not wet, then you must have a leaking rear wheel cylinder that is draining the rear compartment of the master cylinder. If they are not leaking, then must be a leaking rear brake hose or other leak in the rear brake system.

tim21391 · 07-16-2009, 12:51 PM

master cylinder i bought from autozne fore $13.. have had it for almost 2 years and have no problem with it yet... knock on wood

**leddzepp** · 07-16-2009, 01:05 PM

I have the same problem with my 3/4 ton and I have an NOS master cylinder on it. I have never had the problem with my 1/2 ton. Every time I read about this problem on here, it is always a 3/4 ton that has this problem. It sounds to me like the cap seal is not seating properly. I have heard guys recommend lightly sanding the mating surface on the top of the master cylinder to get a better seal from the cap....but I have not tried it yet.

pete29 · 07-16-2009, 01:11 PM

It has new rear cylinders. and its leeaking in the front part where the cap seats aginst the master cylinder. Somebody told me it might be a proportional valve needed adjusting cause it seems like theres to much pressure in the back compartment of the master cylnder. Then it pushes it to the front and builds up to much pressure and sprays out. Can you adjust the proportional valve

grayw0lf · 07-17-2009, 11:06 AM

Been there, done that. Bad master cylinder from AutoZone.

07-16-2009, 12:30 PM	#1
pete29 Registered User Join Date: Dec 2007 Location: Knoxville, Iowa Posts: 122	brake fluid runs from rear of master cylinder to front then out of it I have a 72 chevy 4x4 3/4 ton. The brake fluid runs from the rear of master cylinder to front and then runs out of it under the cap. I put a new cap gasket in. The rear compartment of the master cylinder will run dry. What could be the problem? Brakes work good if i keep fluid in both compartments.

07-16-2009, 12:44 PM	#2
mclairmo Registered User Join Date: Jan 2006 Location: Take Off Posts: 1,908	Re: brake fluid runs from rear of master cylinder to front then out of it If I am understanding your problem correctly, it sounds like maybe a piston seal is leaking and you are losing fluid to the outside. You need a new master cylinder. Check under the master cylinder near the firewall (manual brakes) or power booster to see if it is wet under there. The fluid would have to leak out of the bore at the back of the master cylinder where it meets the firewall or the power booster. If not wet, then you must have a leaking rear wheel cylinder that is draining the rear compartment of the master cylinder. If they are not leaking, then must be a leaking rear brake hose or other leak in the rear brake system.

07-16-2009, 12:51 PM	#3
tim21391 Registered User Join Date: Jul 2007 Location: Salina, Kansas Posts: 1,998	Re: brake fluid runs from rear of master cylinder to front then out of it master cylinder i bought from autozne fore $13.. have had it for almost 2 years and have no problem with it yet... knock on wood __________________ 1972 C-10...402/400..flowmaster 40 series 1967 Mustang 347 stroker/C-4 1966 Mustang 289/3-speed 2013 Mustang V6/6speed w/300 ponies I may be 23, but i sure do knows 67-72s are sexy! Its not MPG its smiles per gallon! build started 11/25/08 build thread: http://67-72chevytrucks.com/vboard/s....php?p=2993796

07-16-2009, 01:05 PM	#4
leddzepp Moderator Join Date: May 2007 Location: Southern Cal Posts: 20,031	Re: brake fluid runs from rear of master cylinder to front then out of it I have the same problem with my 3/4 ton and I have an NOS master cylinder on it. I have never had the problem with my 1/2 ton. Every time I read about this problem on here, it is always a 3/4 ton that has this problem. It sounds to me like the cap seal is not seating properly. I have heard guys recommend lightly sanding the mating surface on the top of the master cylinder to get a better seal from the cap....but I have not tried it yet. __________________ 1972 C/10 Cheyenne Super SWB. Restored, loaded, slammed. 1968 C/10 50th Anniversary LWB. Unrestored, stock, daily driver/work truck. RIP ElJay RIP 67ChevyRedneck RIP Grumpy Old Man RIP FleetsidePaul

07-16-2009, 01:11 PM	#5
pete29 Registered User Join Date: Dec 2007 Location: Knoxville, Iowa Posts: 122	Re: brake fluid runs from rear of master cylinder to front then out of it It has new rear cylinders. and its leeaking in the front part where the cap seats aginst the master cylinder. Somebody told me it might be a proportional valve needed adjusting cause it seems like theres to much pressure in the back compartment of the master cylnder. Then it pushes it to the front and builds up to much pressure and sprays out. Can you adjust the proportional valve

07-17-2009, 11:06 AM	#6
grayw0lf Registered User Join Date: Apr 2004 Location: Clanton, AL Posts: 1,253	Re: brake fluid runs from rear of master cylinder to front then out of it Been there, done that. Bad master cylinder from AutoZone. __________________ 85' CUCV M1009!!! The newest addition! 6.2diesel, Th400, NP208, & only 36k miles! 70' C-10 LWB Fleetside - Looking good these days! 05' Dodge Neon 88' Winner Escape Sport 1750 - 4.3v6 94' Seadoo SP 84' Honda TRX200, bare nekkid. Just a frame & tires. Always looking for another project or any good deal!

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