HELP Brake Master Cylinder-no rear fluid

carnutjw

Master Mechanic
Supporting Member
Sep 17, 2017
474
93
Soooo.... confession time. My Cutlass dash brake light came on one day in November 2021, no leaks or fluid loss, so I assumed the four year old "remanufactured" O'Reilly master cylinder (part #10-1738) had taken a crap. Replaced M/C, tried to bleed brakes but had no fluid coming from rear bleeder screws. My assumption at this point was that the combination valve was stuck off center. Proceeded to remove combination valve. Between cold weather (by my standards), lack of ambition, and rusty fittings on the valve, I finally removed it a couple of weeks ago. Installed new Inline Tube front and M/C lines, along with a CPP PV-2 combination valve, still nothing out back. Loosened fittings at M/C and realized there was no fluid coming from front port (rear brakes). Swapped basically new remanufactured M/C for another from O'Reilly, bench bled, installed. Again, nothing from front port. Am I just getting bad parts or having a brain fart and missing something in my process? Need to buy "new" M/C? Reminder, if it matters, that I have C6 calipers on the front and stock drums out back.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
4,302
113
Upstate NY
Have you tried bleeding the master cylinder by more than one method?

Sounds like the forward piston is stuck.

When in doubt, take the master cylinder pistons/springs/seals apart and see exactly what is happening.
 
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carnutjw

Master Mechanic
Supporting Member
Thread starter
Sep 17, 2017
474
93
Was there fluid coming out of the front port when you bench bled it?
I just did the “recommended” bench bleed-plug the ports, then slow 3/4”-1” strokes, but it never got to the firm at an 1/8” inch spot maybe 1/4”-3/8”. I am going to pull it off and pull the guts out unless someone suggests otherwise. I have some lines made up to do a recirculating bench bleed.
 

Streetbu

Know it all, that doesn't
Supporting Member
May 22, 2011
3,087
113
Central NY
I've never plugged the ports, I've always recirculated them back into the reservoir.
 
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carnutjw

Master Mechanic
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Thread starter
Sep 17, 2017
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93
Yeah, me neither until the M/C instructions said I should ???
 

carnutjw

Master Mechanic
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Thread starter
Sep 17, 2017
474
93
AC416EAD-B43F-4CA5-A501-AF2DBE2D5470.jpeg
 

malibudave

Greasemonkey
Supporting Member
Mar 12, 2010
194
43
Houston, TX
When using the plug method, it lets you know that the master cylinder is holding pressure AND that there is no air.

If you are getting piston movement, then there is air OR the master cylinder is defective.

You can also try bleeding the master cylinder like a caliper: crack both plugs, push the pedal and hold, tighten plugs, release pedal, and repeat.

You can do this bleeding method on the car, BUT you have to make sure the bottom of the master cylinder is level to the ground. The nose cannot be pointing up as air may stay trapped in the master cylinder. You can get it level by jacking up the rear end to a point that the bottom of the master cylinder is level. This will save you having to remove it from the vehicle.
 
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scoti

Royal Smart Person
Supporting Member
Sep 5, 2019
1,486
113
Texas
When using the plug method, it lets you know that the master cylinder is holding pressure AND that there is no air.

If you are getting piston movement, then there is air OR the master cylinder is defective.

You can also try bleeding the master cylinder like a caliper: crack both plugs, push the pedal and hold, tighten plugs, release pedal, and repeat.

You can do this bleeding method on the car, BUT you have to make sure the bottom of the master cylinder is level to the ground. The nose cannot be pointing up as air may stay trapped in the master cylinder. You can get it level by jacking up the rear end to a point that the bottom of the master cylinder is level. This will save you having to remove it from the vehicle.
Never understood how exactly the plugs were effective vs. using the return hoses & purging that way.

I have the plugs stashed in a drawer & still do the loop bleed to this day.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
4,302
113
Upstate NY
I believe the plug system will will evacuate the air on the return stroke. I never used to use that method either, but if the MC is particularly difficult and is due to one of the pistons being stuck, it can help free it up. I have had success with it. But I'm Streetbu , I always use the two line method and resort to the plugs as a last resort.

carnutjw it's not unheard of to get several rebuilt MC bad out of the box. You're using a MC for an old car, how long ago was it made? It could've been a decade ago for all you know and it doesn't take much to go wrong to cause an issue. If can't be bench bled, there is no reason to put it on the car to confirm that it doesn't work.

My $.02
 
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