GMT800 master cylinder swap problem

There is a shiny new master cylinder from a 2005 Silverado hooked up to my booster. All connections are tight and all 4 corners bled. The pedal feels way better. The braking feels different, but I also swapped in a 14 bolt semi floating rear axle with bigger drums at the same time. Perhaps some of the different feeling is from the bigger drums/shoes.

At any rate, I had to lay in to the brakes pretty good today and I was not pleased with the results. The brakes seemed to pulse and shake the whole truck something fierce.


Anyone have some experience or insight? What did I do wrong?

warped drums and rotors?

For drum brakes to work right they need 10 psi residual pressure . disc brakes need 2 psi. the nbs have disc in the back

The brakes are fine otherwise. Very smooth, but either it's my imagination or the bias is slightly different. This is why I remarked about it feeling a little different. It seems like the truck noses down as I apply the brakes where maybe it wasn't as noticeable before. The odd shaking has only occurred twice, each time I nailed the pedal. It left me wondering if the ABS was unhappy with something.

I don't really understand the residual valve, after reading it seemed that it's necessary to keep the shoes from opening too far when the pedal is all the way up.

Are the lines supposed to hook up as they were with fore line to fore port and aft line to aft port? I read that the SSBC MC upgrade instructs for the lines to hook up reversed. The SSBC MC looks a lot like the GMT800 MC.

I read somewhere there is a suburban specific TSB for a proportioning valve.

beardeddummy said:

For drum brakes to work right they need 10 psi residual pressure . disc brakes need 2 psi. the nbs have disc in the back

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I put a NBS MC on my 90, and I just never thought about needing a residual valve. And since then I have been too lazy to put one on. Stops great and haven't experienced the awfulness he reported. Not saying that's not the problem, or that one should not install a res. Valve.

Well if i ever get my truck driving I'll let you know. I got a gmt400 hydroboost with a 800 MC and a 400 style prop valve. Dana 60 with single piston discs front and a dana70hd drums rear

It's coming off. Seems like nothing more than a novelty. Going back with the old MC and a set of 1 ton calipers and pads.

Could just temporarily disable the ABS and see if it stops the shaking? Not that it's your issue, but i've had vehicles where under normal braking they felt fine, but under hard braking, they would get a little hotter and feel warped. The next time stopping, they felt normal.

Trust me, this was no instance of warped hardware. This was something else, out of my league, and I'm ditching it.

slowburb said:

It's coming off. Seems like nothing more than a novelty. Going back with the old MC and a set of 1 ton calipers and pads.

Click to expand...

You can not use 3/4 or 1 ton pads on a half ton rotor hat without it rubbing, unless you take a grinder to the pad backing or machine down the rotor hat diameter.

On top of that, the 3/4 and 1 ton calipers will require more fluid volume to move the pads the same amount.

Likely you will not like that result either, longer pedal travel.