Brake Master Cylinder won't pump up after rebuild.

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Halfdunn
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Brake Master Cylinder won't pump up after rebuild.

Unread post by Halfdunn » May 13th, 2018, 2:45 pm

Hi all, any help with the M151 master cylinder not maintaining pressure after rebuild kit installed. All parts were replaced in same order as removed, precharged the unit before install and have bled all wheels to remove any bubbles. no leaks observed after rebuild.

Pumps up okay but won't hold after release.

Looking for a diagram of the actual master cylinder parts breakdown and cannot find one even in the Tech manuals for parts.

Thanks,

Halfdunn

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rickf
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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by rickf » May 13th, 2018, 4:30 pm

Was there any rust in the bore of the cylinder and did you hone the bore? It sounds like the piston cup got scored or cracked.
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Mr. Recovery
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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by Mr. Recovery » May 13th, 2018, 4:31 pm

can't find a diagram right now so , here's a few photos of the last one I rebuilt.

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Halfdunn
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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by Halfdunn » May 13th, 2018, 6:44 pm

Thanks everyone for your assistance and pictures. My current M/C(Master Cylinder) was brand new with no scoring or rust encrusted parts. It would pump up but not hold. Depressing the brake pedal after resting momentarily, resulted in it going to the floor. I put a rebuild kit in the new M/C to no avail. Still would pump up but not hold after release going to the floor again. That is why I wanted to see a diagram to ensure parts were in the proper sequence in the M/C. I did install the parts of the rebuild kit in the M/C just like they were removed from the original M/C. Still bleeding the system at all wheels and no relief.

Halfdunn

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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by Hambone » May 13th, 2018, 9:13 pm

Halfdunn
Check out this video, it has a cutaway that shows how it is assembled, if needed, I can break one down for you.
https://www.youtube.com/watch?v=c3uNjEcICSw

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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by rickf » May 13th, 2018, 9:39 pm

You have a problem with the front piston cup, either it is scored, torn or put in backwards. The fact that you can pump up the brakes would tell me it is installed correctly but damaged.. Air in the system would not cause as much problem as you are having after so much bleeding. You are following a proper bleeding procedure aren't you?
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Halfdunn
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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by Halfdunn » May 13th, 2018, 10:24 pm

Thanks rickf.

Video cutaway shows my parts installed appropriately on my M/C. Following normal bleeding procedures as in the TM for brake bleeding. Will review cups for damage but did that prior to install. Both looked good with no cuts, scoring, etc.. M/c bore was clean with no damage from visual inspection.

Will attempt a redo of bleeding and see if that works.

Thank you again.

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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by miljeep » May 14th, 2018, 2:48 pm

Make sure you have the correct cup, I bought a re-build kit a few years back from a M151 parts dealer and he sent the wrong cup.... Scott O&O

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muttguru
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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by muttguru » May 14th, 2018, 3:41 pm

There's some interesting information about master-cylinder problems in an old series of posts which you can find here:-

http://forums.g503.com/viewtopic.php?t=289691

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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by m3a1 » May 14th, 2018, 5:50 pm

And I'll throw this idea out there for good measure...

Are you SURE you have no leaks elsewhere?

Halfdunn
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Re: Brake Master Cylinder won't pump up after rebuild.

Unread post by Halfdunn » May 14th, 2018, 10:07 pm

Thanks again for all of your suggestions. Previous threads were helpful. I believe all of my parts are installed properly but I will check the size of the new cup. No leaks observed any where else in the system and fluid only needs adding after bleeding. Everything is still dry even after all of the bleeding process.

I believe bench bleeding is what I did prior to install by filling reservoir and pumping the piston until only fluid comes out of the outlet.

Again, you guys are the best and I truly appreciate all of the assistance.

Halfdunn

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