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

greg72 · 05-20-2003, 02:37 AM

I just got the truck up and running but I have beld the brake on three seperate occasions. Each time I get almost like a vacuum in the master cylinder where after two bleeds, I get no more fluid from the right rear. I replaced the wheel cylinder and bled this time with the master cylinder with the lid off. Again after three days the brakes got spongy and the side going to the rear brakes was almost empty again. I can see no leaks at all. What about the proportioning valve? Is it possible to have an air lock inside it? Is there a procedure to bleed it or test it? I am stumped.

JimKshortstep4x4 · 05-20-2003, 07:55 AM

There has to be a leak somewhere in the system. Do you have power brakes?

It is possible to have the master cylinder leak out the back and into the brake booster so that the fluid is not visible.

Jim

greg72 · 05-20-2003, 12:45 PM

Does this look right??

greg72 · 05-20-2003, 12:46 PM

here is another. Should the M/c lean forward this much?

greg72 · 05-20-2003, 12:50 PM

Yes it is power brakes. This is the weirdest thing. I will check into the leak into the vacuum booster. The bleeding is weird. I get two good shots of fluid from the right rear. On the third bleed, I get nothing until I remove the lid from the m/c. Kind of like it creates a vacuum.

I bled with the lid off once and I got better bleeding results at the right rear.

I can't explain the loss of fluid however. I have filled the front m/c reservior three times.

JimKshortstep4x4 · 05-20-2003, 08:49 PM

It appears that you have a 3/4 ton setup. I was thinking of the 1/2 ton which has the rear brakes fed off of the rear reservoir. Your leak has to be in the lines to the rear or the rear wheel cylinders.

Sometimes rear brakes can be painfull to bleed. I have bled the mastercylinder by removing the lines exiting the proportioning valve and then bleed the rear wheel cylinders.

As long as you are losing fluid you will have problems bleeding the system. I would reexamine everything from the prop valve back for leaks.

Good luck!

JIm

greg72 · 05-21-2003, 01:00 AM

I got this set up from a salvage yard truck complete. I have read in the service manual to hold the combination valve "open" when bleeding the brakes. Any experience out there with this?

05-20-2003, 02:37 AM	#1
greg72 Registered User Join Date: Dec 2002 Location: Central California Posts: 88	Brake Help I just got the truck up and running but I have beld the brake on three seperate occasions. Each time I get almost like a vacuum in the master cylinder where after two bleeds, I get no more fluid from the right rear. I replaced the wheel cylinder and bled this time with the master cylinder with the lid off. Again after three days the brakes got spongy and the side going to the rear brakes was almost empty again. I can see no leaks at all. What about the proportioning valve? Is it possible to have an air lock inside it? Is there a procedure to bleed it or test it? I am stumped. __________________ Greg72

05-20-2003, 12:45 PM	#3
greg72 Registered User Join Date: Dec 2002 Location: Central California Posts: 88	here are a couple of pics Does this look right?? Attached Images __________________ Greg72

05-20-2003, 12:46 PM	#4
greg72 Registered User Join Date: Dec 2002 Location: Central California Posts: 88	another pic here is another. Should the M/c lean forward this much? Attached Images __________________ Greg72

05-20-2003, 12:50 PM	#5
greg72 Registered User Join Date: Dec 2002 Location: Central California Posts: 88	more info Yes it is power brakes. This is the weirdest thing. I will check into the leak into the vacuum booster. The bleeding is weird. I get two good shots of fluid from the right rear. On the third bleed, I get nothing until I remove the lid from the m/c. Kind of like it creates a vacuum. I bled with the lid off once and I got better bleeding results at the right rear. I can't explain the loss of fluid however. I have filled the front m/c reservior three times. __________________ Greg72

05-21-2003, 01:00 AM	#7
greg72 Registered User Join Date: Dec 2002 Location: Central California Posts: 88	thanks for the help I got this set up from a salvage yard truck complete. I have read in the service manual to hold the combination valve "open" when bleeding the brakes. Any experience out there with this? __________________ Greg72

05-20-2003, 07:55 AM	#2
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	There has to be a leak somewhere in the system. Do you have power brakes? It is possible to have the master cylinder leak out the back and into the brake booster so that the fluid is not visible. Jim

05-20-2003, 08:49 PM	#6
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	It appears that you have a 3/4 ton setup. I was thinking of the 1/2 ton which has the rear brakes fed off of the rear reservoir. Your leak has to be in the lines to the rear or the rear wheel cylinders. Sometimes rear brakes can be painfull to bleed. I have bled the mastercylinder by removing the lines exiting the proportioning valve and then bleed the rear wheel cylinders. As long as you are losing fluid you will have problems bleeding the system. I would reexamine everything from the prop valve back for leaks. Good luck! JIm

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