Master Cylinder Upgrade?

[AvgJoe]

Let me start at the beginning. The rear brake line popped on my way home from work one day from age and I managed to limp her home. The line was popped along the frame rail behind the fuel tank. It was pretty rusty. That was a relatively easy repair. I was trying to bleed the brakes and realized there was no fluid coming from the rear bleeders. I figured it was the ABS dump valve holding back. Wanting to do away with the ABS anyway, I made up some new lines and tried again. Still no fluid. Then I found my proportional valve had done its job in a sense and diverted all the fluid to the front wheels during the loss of pressure. However it had seized in that position. That’s when I found out it is an obsolete part now. I started doing some research on the GMT800 MC upgrade. I bought that MC but had to use a proportional valve from an older 3rd gen truck with disc/drum. Here is the predicament I’m in: The brake pedal is firmer but the truck doesn’t want to stop. None of the wheels will lock up. They act as if they are glazed but even if they are, I should be able to get one them to lock. Neither front nor rear will lock. And yes, the rear brakes are adjusted. I can’t see all four corners glazing at the same time. I haven’t pulled the calipers off yet and won’t have the time to do so until after the holiday. Both the pads and shoes have about 80% of their life left. I have another question about that NBS master cylinder. Which port is for the rear brakes? I have it hooked up now as the OBS style would have been but is that right? Or could that be my problem? I’ve been at this for a while so any help would be appreciated.

 

[evilunclegrimace]

Is the push rod in the booster the correct length for the new Master cylinder?

 

[AvgJoe]

The vacuum booster is factory GMT400 and the cylinder is GMT800. There was no mention in any thread I read about changing the booster during the upgrade. "All you need is a NBS MC and a fitting" to convert the one port to correct size. I can't change the push rod length.

 

[evilunclegrimace]

The vacuum booster is factory GMT400 and the cylinder is GMT800. There was no mention in any thread I read about changing the booster during the upgrade. "All you need is a NBS MC and a fitting" to convert the one port to correct size. I can't change the push rod length.

Yes you can change the push rod. Refer to section 5 in the GM unit repair manual. You need a J-22647 gauge to measure the push rod length. And just to be clear, we are talking about the push rod that goes between the booster and the Master cylinder not the rod between the pedal and the booster. If the push rod is to short you will not get full master cylinder piston travel and if the push rod is to long it will not allow the Master cylinder piston to return to its fully seated position.

 

[AvgJoe]

I'm confused? So my truck is the only one that can't have the GMT800 master cylinder installed without the booster push rod change? Where do I get a push rod for a booster without buying a booster? And for what year truck do I buy a booster? I have read numerous threads all a bunch of truck forums about this upgrade but none of them mention booster push rods. Some people report an improvement while others do not. I had to change my proportional valve also which I think may be a bigger issue than the push rod. Also, I still don't know for sure which port on the NBS unit is for the rear. I think I'm going back to the OBS with the 3rd gen prop valve and hope for the best. I don't know what else to do.

 

[magnumb]

If you are going back, why not try a JB7 master as it uses the same booster and you don't need any adapters. Then upgrade to the non-low drag 3/4 1 ton calipers.

 

[evilunclegrimace]

I'm confused? So my truck is the only one that can't have the GMT800 master cylinder installed without the booster push rod change? Where do I get a push rod for a booster without buying a booster? And for what year truck do I buy a booster? I have read numerous threads all a bunch of truck forums about this upgrade but none of them mention booster push rods. Some people report an improvement while others do not. I had to change my proportional valve also which I think may be a bigger issue than the push rod. Also, I still don't know for sure which port on the NBS unit is for the rear. I think I'm going back to the OBS with the 3rd gen prop valve and hope for the best. I don't know what else to do.

I looked up this swap and I found that some of the folks doing the swap did have push rods that were up to a 1/4" in length different.

The 800 master is shorter overall than the 400 master,so it is entirely possible that if the push rod is longer it is keeping the piston partially applied. This could be causing the glazing condition, you need to do some measuring.

I installed a booster and master from an 1988 K-3500 on my K-1500 and also swapped the hub/bearing units and rotors to a F-44 front brake option from an extended cab K-1500 and my brakes are very much improved. I plow with my SCSB and it was scary to try and stop with the plow on before i made the changes,

 

[shamrock246]

I did that swap to the NBS master but didn't like it. I will most likely go to the 3/4 master and calipers from the JB7 system and hydroboost it if not happy after that.

This a good read
http://www.gmt400.com/threads/jb5-jb6-jb7-jb8-size-thread.39647/