91 GMC z71 - weak brakes?

I'd try replacing the combination valve..........

Doc

Ugh, hit "send" too soon. You could also look into the front flex hoses..............might be internally collapsing at the same rate hence no pull but reduced stopping power.

Doc

When you replaced the rear shoes, did you adjust them properly? To eliminate them as the cause of your problems, adjust them tight against the drums(can't turn wheels) and check the pedal feel. If the pedal comes up substantially, they may have been too loose. H

"Hamilton Audio" wrote

And "many" are right.

Ian

Hey doc,

sorry to sound stupid, but what is a combination valve? is it like a proportioning valve? I saw you refer to it a few days ago, but i've never heard of that part?

thanks TB

The combo valve on the GM trucks combines a proportioning valve and metering valve in one package. It's located just to the right of the master cylinder and is aluminum. The lines travel from the master to the combo valve, then to their final destination (or ABS module if so equipped).

Doc

And a metering valve does what? I too was only familiar with the proportioning valve (enquiring minds want to know...;>)

Proportioning valve controls the amount of pressure applied to front and rears based on vehicle load; the metering valve holds off application of the front brakes until the rears have a chance to kick in. Rears have more "travel" before they apply, so the metering valve holds off the fronts until the rears are applied to reduce the nose dive effect.

Doc

Gotcha. I guess I always thought the proprtioning valve did both, plus on my Blazers I've never had much luck with the rears in general...thanks.

Re-Bleed the system. The GM manual says the two man method is recomended, with ~2 quarts for a good bleed. IMHO, I believe there can be air trapped in the ABS system. I had blead mine to the point that air was sucked in to the system. It wasnt right until I blead the system 3 times. And do check the rear adjustments.

Cheers

Proportioning valve controls the amount of pressure applied to front and rears based on vehicle load;

-AKA height sensing valve usually attached to rear axle limiting apply pressure to rear brakes when truck empty and maximizing apply press. when truck loaded. Not all are so equipped!

The other part of the "combo valve" triggers the brake warning light when fluid is lost from either front or rear circuit.

agreement on delay front apply function

Rolf

the front flex hoses were brand new when I bought the truck 3 years ago....could it be that they've deteriorated already?

Nah, not if they're that new............

Doc