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 Is it the master cylinder?

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Posts : 129
Join date : 2015-05-27
Age : 35
Location : Hollywood, Florida, USA

PostSubject: Is it the master cylinder?   Fri Feb 08, 2019 2:14 pm

Hi all,

Here's hoping my story will help one of you guys from banging his head too hard at his brakes.

A couple weeks ago, I decided to check my rear drums because the car was pulling left when I hit the brakes, but it wasn't pulling the wheel. No biggie I thought. I pulled the right side drum and it was caked in grease! The axle seal had failed. The seal failed because the axle bearing had started eating the axle. Fun.

Ok, so now ordered new seals, bearings, full rear brake kits(springs,shoes,cylinders,drums), and a new axle. Spent a weekend putting it all together, marveling at how easy pulling the axles was, and thanking the GM engineers that made knocking the differential pin too far through didn't require drilling out the pin. Put it all together, and all seemed well. Brakes seemed a little shuddery (Current manufacture drums seem to all be terrible), so I pulled the Napa set, and ordered the ACDelco drums. Much better! Odd though, they get shuddery after a drive. They would shudder while braking, and keep going while rolling. Felt like a wheel/driveshaft imbalance that would get better if you accelerated briskly.

I started taking an IR thermometer with me to check temperatures. After driving to work a couple times (30miles each way @ 60-70mph) the rears would get to around 205F-210F while the fronts were only showing 135F-145F. Weird. I chalked it up to the shoes were still bedding in and would be hot and odd until they would bed properly.

A week later, they're still doing it, only a little better. I finally decided to check my master cylinder to see if it was leaking into the brake booster.

Sure enough, it was, but it was a very slow leak, pedal was always firm, level dropped very slowly (months between top ups of the rear(front brake) section, thought it was the calipers moving). I got a new master Dorman brand box that was labeled Wagner MC. Bled it with Dot3, and mounted it up.

What a difference! Car has a bit of nose-dive on hard braking, and the temperatures are now after hard use 201F on the front calipers and 150F on the rear drums! This car has had poor/ok-ish brakes since I'd gotten it around 3 years ago. It got new calipers,rotors,drums etc but never checked the master. Wish I would have started there!

TLDR: If the brakes are overheating in the rear, or if it seems to take too much pressure to stop, or stay stopped at a light, check the master cylinder.

I really wasn't looking forward to the very shaky run to Wagonfest!
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Fred Kiehl

Fred Kiehl

Posts : 4992
Join date : 2009-11-13
Age : 70
Location : Largo, FL 33774

PostSubject: Re: Is it the master cylinder?   Fri Feb 08, 2019 10:33 pm

Glad you found the issue. Another problem item can be the proportioning valve. I had one fail. The reservoir in the front would fill the rear reservoir, and the braking was weak. The proportioning valve is not available new, or rebuilt.
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