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  View original topic: cracks in master cylinder reservoir.
hiram6 Sun Mar 14, 2010 3:25 pm

I'll attempt to get some pics up, but I have a new reason to hate 25 year old plastic. As the final step in my rear disc brake install, I hooked up my Motive Power Bleeder today to bleed and flush my brake lines.

If you are familiar with the Motive power bleeder, you know it works by applying pressure at the master cylinder to force fresh fluid through the master cylinder and through the brake lines.

I hooked all the connections up and pumped up the system to 10 pounds pressure. Went out back to crack open the bleeder on the right rear.

Went back up to the master cylinder, just to check and ...........holy crap!!!! Brake fluid dripping everywhere under the dash!!

Released the pressure on the power bleeder and cleaned everything up. So where did I go wrong? I hooked all back up again, and sloooooooowly pumped up the pressure.

Then I saw it. Brake fluid seeping through the sides and top of the brake cylinder reservoir through age cracks. A spider web of tiny cracks that apparently don't leak under the pressure of daily use, but the pressure of the power bleeder was too much, even at less than 5 pounds pressure.

I'll find a good used one, just putting it out there that you may want to inspect that 25 year old plastic that's been sitting under your dash, under the heat from that big windshield.

hiram6 Sun Mar 14, 2010 4:44 pm

Okay, here are some pics.

Top of reservoir:







Hard to get a good shot of the sides, but you can see some of the cracking here.




When you press on the top with your thumb you can hear a creaking/cracking sound. Sounds like it's just brittle from the heat.


Anyway, maybe I have the only one out there that has done this, but something else to watch out for.

Dingchowping Sun Mar 14, 2010 5:17 pm

That's a bummer. I don't envy you the task of cleaning up all that brake fluid. Its not just 25 year old plastic that fails like this. Early B5 Audi A4s and VW Passats with round master cylinders suffered similar cracking after just 5-7 years in service. Using a power bleeder would cause the reservoir to rupture, spraying brake fluid all over the windshield. One of many reasons why I stopped using power bleeders.

On a side note I recently discovered something rather odd about the new Audis...after having a bear of a time getting a decent bleed on an RS4 I looked up the factory procedure and they tell you to start with the bleeder closest to the reservoir and move progressively to the furthest...opposite of well established procedures. I have no idea why...maybe there's something unusual about the 8 piston calipers on the RS cars, or the electronic brake biasing, but whatever the reason it solved my problem. Weird.

Alaric.H Mon Mar 15, 2010 2:11 am

Yep me too I disconnected the battery and put a hose on it and then blew it with air and vacuumed it all up not the high light of my day.


hiram6 Mon Mar 15, 2010 4:47 am

Dingchowping wrote: That's a bummer. I don't envy you the task of cleaning up all that brake fluid. Its not just 25 year old plastic that fails like this. Early B5 Audi A4s and VW Passats with round master cylinders suffered similar cracking after just 5-7 years in service. Using a power bleeder would cause the reservoir to rupture, spraying brake fluid all over the windshield. One of many reasons why I stopped using power bleeders.

On a side note I recently discovered something rather odd about the new Audis...after having a bear of a time getting a decent bleed on an RS4 I looked up the factory procedure and they tell you to start with the bleeder closest to the reservoir and move progressively to the furthest...opposite of well established procedures. I have no idea why...maybe there's something unusual about the 8 piston calipers on the RS cars, or the electronic brake biasing, but whatever the reason it solved my problem. Weird.

Yeah, cleaning up sucks, I'm tempted to pull the dash and really do it right, since I have some other things to do in there eventually. I have a replacement heater core to do, although I was hoping to wait until fall to deal with that, and some other wiring to clean up in there. The previous owner got a little creative in there tapping into circuits, and I'd like to straighten out some of the irregularities. Like for instance wiring the blower switch so the blower turns off with the ignition, which would be a nice touch.

You know, since the cracks don't seep except under the pressure of the power bleeder, I'm tempted to clean the hell out of that thing, slather the exterior with a good coat of plastic epoxy, and stick it back in there. Using a vacuum bleeder at the calipers should work okay, and put no pressure on the reservoir. 95% of the cracks are above the MAX indicated mark anyway.

Terry Kay Mon Mar 15, 2010 5:20 am

Pressure bleeder's have thier place--with cast master cylinder's, not plastic.

I've always used the gravity bleeding system--open the bleeder's and walk away for 5 minutes--just keeping an eye on the brake fluid level.

But, on the other hand, maybe you did yourself a favor and located a problem in the cracking of the reservoir.

randywebb Mon Mar 15, 2010 12:27 pm

yes, you found a failing reservoir

brake fluid is a hazardous waste & takes off paint like crazy

I'd be inclined to clean it up really really well - and immediately. Use brake cleaner (right?)


there are little suction guns to pull brake fluid out also - maybe called Might Mite or some such

Good Luck

buildyourown Mon Mar 15, 2010 12:32 pm

randywebb wrote: yes, you found a failing reservoir

brake fluid is a hazardous waste & takes off paint like crazy

I'd be inclined to clean it up really really well - and immediately. Use brake cleaner (right?)


there are little suction guns to pull brake fluid out also - maybe called Might Mite or some such

Good Luck

Brake cleaner can also harm plastic and paint.
Isopropal Alchohol will lift the brake fluid but won't harm other plastics.



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