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chevygeorge, ASE Certified Technician
Category: Chevy
Satisfied Customers: 53
Experience:  GM Technician for over 25 years
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Gmc master brake cylinder bubbling up in first reservoir

Customer Question

gmc master brake cylinder bubbling up in first reservoir next to fire wall
Submitted: 3 months ago.
Category: Chevy
Expert:  chevygeorge replied 3 months ago.

Hello, My name is ***** ***** I'm here to assist you. The rear reservoir(closest to firewall) is for the front brakes. It is not uncommon to get some fluid movement(a stream) from the reservoir when applying the brakes. If you are getting bubbles, then you have air in the front brake fluid circuit.

Customer: replied 3 months ago.
no air just fluid I cannot seem to be able to build up brake pressure
Expert:  chevygeorge replied 3 months ago.

Are you getting a red brake light and no pedal? Have there been any recent repairs before the problem started? What year is the truck? (it says pre-1983)

Customer: replied 3 months ago.
Customer: replied 3 months ago.
it is a1981 there is no pressure to the back brakes new master and all new calipres
Expert:  chevygeorge replied 3 months ago.

I had an '80 Big 10 that would get intermittently a spongy pedal. I would bleed the rear brakes and it would be fine for a couple years, although I never got a large volume of fluid during the bleed. I suspected air was entering the system through the aftermarket wheel cylinders intermittantly. Is the pedal low, in addition to no pressure to the rear brakes. Are the shoes physically not applying?

Expert:  chevygeorge replied 3 months ago.

The proportion valve can stick, but will usually cause the red brake light to come on, unless it has been unplugged at the valve. The valve is located on the driver's side front frame rail near the gear box. Sometimes tapping the valve will unstick it. It used to be a discontinued part, but may be available from restoration companies now.

Expert:  chevygeorge replied 3 months ago.

Have you bled the system at the master cylinder? That would verify you are getting pressure out of the master cylinder. Should be getting a good stream from both lines when holding pedal and cracking line. The smaller front reservoir is for the rears. I have seen new parts not function properly(especially master cylinders) so don't rule that option out.

Customer: replied 3 months ago.
the back brakes have been changed to calipers and they are not moving
thanks I think because it was setting so long the proportioning valve is sticking
thanks harold
Expert:  chevygeorge replied 3 months ago.

It is a common issue with the valve. It may need a different valve to compensate for the disc swap, also.

Expert:  chevygeorge replied 3 months ago.

Please take the time to rate my service and feel free to let me know if you need any further assistance. Thank you, George