Blew out the MC…

[SoCalSK8r]

I was bleeding my brakes using a pressure bleeder and my master cylinder failed, that is a mess that was less than fun to clean up.

I suppose it’s better than failing in the canyons.

Public service announcement, replace your 30+yr old master cylinder.


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[fanaudical]

How exactly did your master cylinder fail? Did the reservoir fail? Or did the master cylinder seals fail somehow?

I would think the master cylinder is capable of developing much higher pressure than your pressure bleeder can...

[Uwon]

As fanatical said, did your reservoir fail?..OR… the lines/fittings into the MC. Does not take much pressure before either start leaking. Do it the old way by pumping the pedal or if you are patient, by gravity alone (may have to urge flow with minimal initial pumping).
Cheers,
Johan

[mikesarge]

Had this happen with my Vanagon recently when bleeding the new clutch master. It lead to a whole hydraulic system overhaul.

Massive PITA to clean and deal with. I don’t have any advice, but misery loves company.

[shamrok]

Not that this is the culprit, but when using a pressure bleeder, there's no need to use too much pressure. I think I use only 10-15 psi with my Motive.

[MrBonus]

Secondary advice, ensure your emergency brake is in functioning order.

[tasturbo86]

Use a vacuum bleeder that works off of a compressed air line.
I vacuum the brake fluid reservoir, and then fill it with new fluid.
Then you bleed the brakes, starting with the wheel farthest from the master cylinder.

[hcoles]

Originally Posted by shamrok

Not that this is the culprit, but when using a pressure bleeder, there's no need to use too much pressure. I think I use only 10-15 psi with my Motive.

I also use 12-15 psi. I think it is in the instructions somewhere.

[SoCalSK8r]

Let’s see..I pumped it to about 14 psi when the pressure all of sudden dropped and a massive amount of liquid starting coming from the bottom of the car.

It is possible I only dislodged a line….I haven’t gotten to the point of pulling it out to inspect as I spent the rest of my time cleaning up the mess best I could.


It’s so annoying though, as I was in the home stretch to wrapping up my front wheel bearing replacement, which took entirely too long. I’m thinking, ok 30-40 more minutes and I’ll have wheels back on to go cruising, WRONG…Murphy decided to show up instead.


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[wazzz]

Make sure you squeeze the upper line coming from the fluid reservoir before applying pressure, as this is an overfill line that runs to the front of the car under the front valence and will spill on the floor. Ask me how I know.

[bahman]

I had a similar experience with my ‘72E. I pressurized the system to 12-14 psi, and both flex lines that feed/connect the reservoir to master cylinder popped out of their grommets and dumped the entire reservoir fluid on the garage floor.

I did not have the physical strength in my fingers to reattached the hoses, and had to remove the MC to get a better access/grip and reconnect the lines. I also replaced the grommets, even though they seemed fine.

[Locker537]

It sounds like you failed to block off the overflow line. If the fluid is leaking from in front of the driver's front wheel, out of a small tube, then that's it.

[SoCalSK8r]

OK, I feel like a giant dumb@ss but thanks to all the folks chiming in. I did not block off the bypass line, so that is most likely the issue.

When I first saw fluid coming from under the car, I immediately grabbed water and rinsed the area around the MC, which made a larger mess thus making it hard to pin point the issue.

I’ll try again by pinching off the tube, you guys saved me a bunch of time with your help, much thanks!!


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[Locker537]

Originally Posted by SoCalSK8r

OK, I feel like a giant dumb@ss but thanks to all the folks chiming in. I did not block off the bypass line, so that is most likely the issue.

When I first saw fluid coming from under the car, I immediately grabbed water and rinsed the area around the MC, which made a larger mess thus making it hard to pin point the issue.

I’ll try again by pinching off the tube, you guys saved me a bunch of time with your help, much thanks!!


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We learn from our mistakes.

Many people, including myself, do not put fresh fluid in the Motive pressure bleeder. Use it dry and pressurize the brake system to ~10 PSI.

1. Block off the overfill hose. I use a little piece of hose and a clamp instead of continually clamping the line on the car. Not necessary, but I like it.

2. Fill the reservoir with fluid.

3. Attach the Motive bleeder without fluid in it. Pump to 10 PSI.

4. Bleed each brake. You will likely need to add more fluid to the reservoir a couple times as you move from wheel to wheel.

This process is a bit slower, but requires less cleanup and likely wastes less fluid.

[SoCalSK8r]

As “luck” would have it, the reserve over flow line was not connected to the drain pipe, which caused some fluid to spill into the cargo area, then drain out through a few holes in the floor pan.

That was a dang mess….but no meaningful carnage.

Dang you Murphy!!!


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[djpateman]

If the reservoir overflow line was not blocked off, then how could you have any pressure in the reservoir? It acts like a vent essentially. But certainly, too much pressure in the reservoir can blow the feed lines from the reservoir into the master cylinder out of the grommets. If the reservoir has the internal wall dividing the front an rear sections, and both are empty, then likely both lines blew out, but if only one side is empty, then only one popped out enough to leak. You do need to ensure both are properly seated.
When bleeding you must stop and check the fluid level constantly to ensure the reservoir never empties, or you have to start again. OH, and DO NOT use so much pressure!
You are right of course, master cylinders do need regular replacement. Back in the 1970's and 80's we replaced them about every 10 years due to leaks; mostly from the rear seal.