Master cylinder help

[thatguyHoBo]

After some time of having braking issues I belive it to be my master cylinder. Shoes and hardware replaced in the rear, pads and rotors replaced up front. Fluid flushed and system bled. Would feel good for a week, maybe, then pedal would start to soften up. Les schwab even verified the functionality of everything and said there was supposedly nothing wrong. After closer inspection I've noticed what appears to be staining on the master cylinder, as well as maybe some staining on the booster? I'll attach some pictures for reference. My problem is that the master cylinder on my truck has a part number that doesn't match my truck according to AutoZone and the service sticker codes are somewhat unrecognizable. Can anyone help determine what brake code i have for a replacement MC? Ill attach pictures of what I have, if anyone can help I appreciate it.

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[tpass]

in between G80 (posi rear end) and KC4 (i dont know what that is) Is the code JB6 for your brakes, check out this thread for more info:

JB5 JB6 JB7 JB8 Size Thread

Hello, After hours of looking around i can't find a thread that list all the sizes of the various brake components of JB style brakes. Please let me know if what i have so far is correct. I'm sure i have things wrong as all info is coming from other threads. JB5(6400lbs): 1.574" Bore MC Low...

www.gmt400.com

 

[Schurkey]

WHAT VEHICLE IS THIS???
Seems to be an '88--'90 half-ton, based on the little bit of the (iron?) RWAL device in the photo. An iron-bodied RWAL should have it's own bleeder valve--did you bleed the RWAL unit?

I concur, looks like JB6 to me.

JB6 should have a step-bore, 3-chamber master cylinder, 29-ish millimeters (1 1/8") regular bore, 40mm rear bore, to go with the low-drag calipers in front. What you have doesn't appear to be a step-bore master cylinder, but maybe that's just due to camera angle. I'd have expected a larger, flared-out casting towards the rear. Maybe yours is, and I'm just not seeing it right.

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[GoToGuy]

If you don't have service manuals, there free here, search in forum for link. Did you look at the photo expanded, you can read the rpo code. JB6 seen this it's in the service manual. Also, you can go to " GM parts giant.com " a very good GM parts site. Enter your VIN , then select categories to get to the parts. The parts may no longer be available, but the parts are still listed, and there good, AC Delco, and others use GM numbers or can cross over. it's a GM database it knows what your truck was built with. This is out of GM service.

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[Erik the Awful]

Would feel good for a week, maybe, then pedal would start to soften up.

Take it to a good brake specialist shop. It really sounds like you have a small leak letting air into the system.

 

[thatguyHoBo]

It's a 91 k2500 6 lug.
I do agree, it looked like JB6 in the service sticker just wanted a second opinion.
I agree with the small leak, I believe it's inside the master cylinder. I see no fluid anywhere else along the braking system.
However it does have the rear abs or rwal and I have not bled that. Not sure if the brake shop I took ot too did or not.
I'll check out the gm parts website listed above. Thanks everyone for the help.

 

[thatguyHoBo]

WHAT VEHICLE IS THIS???
Seems to be an '88--'90 half-ton, based on the little bit of the (iron?) RWAL device in the photo. An iron-bodied RWAL should have it's own bleeder valve--did you bleed the RWAL unit?

I concur, looks like JB6 to me.

JB6 should have a step-bore, 3-chamber master cylinder, 29-ish millimeters (1 1/8") regular bore, 40mm rear bore, to go with the low-drag calipers in front. What you have doesn't appear to be a step-bore master cylinder, but maybe that's just due to camera angle. I'd have expected a larger, flared-out casting towards the rear. Maybe yours is, and I'm just not seeing it right.

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It's a 91 k2500 6 lug.
Replaced the master pn NMC1111 from oreilly. I'm told this part replaces the 10-2352 mc. bench bled then bled at the lines, then at the wheels, farthest from the mc to closest. I used the hose in brake fluid pumping the pedal slow, I was by myself and didn't have my vacuum bleeder.
I then had a wheel cylinder go out. I didn't fully lose breaking but it definitely got worse. Got it home and replaced both wheel cylinders, new shoes, and my drums were glazed so I replaced those. Bled the same as before. Pedal still feels god awful.
Front rotors are relatively new and clean. Pads we replaced same time rotors were and still look to have some life.
My dump valve has a "bleed" screw similar to the prop valve. Is this what I would use to bleed to dump valve? It looks to be aluminum not iron bodied.

 

[Schurkey]

If it's aluminum, you need a scan tool to bleed it.

VERY recommended.

 

[thatguyHoBo]

If it's aluminum, you need a scan tool to bleed it.

VERY recommended.

Heard. I'll try talking my cousin in to it. He was adamant my older truck didn't have that capability.