91 GMC z71 - weak brakes?

I've asked this before, and got a few differnt answers...now I'm curious again. why do the brakes in my truck feel so darned weak????
Many have said this is "just a chevy brake system". I don't buy that. I've
upgraded with fresh rotors and calipers (factory replacements), Raybestos Brutestop pads up front, and regular replacement shoes in the rear. have completely bled the brake system of old fluid and replaced with fresh, clean stuff and re-bled.
Is it possible that after 300,000 km, the old girl's brake booster is getting weak? it seems the pedal has a little bit of feel, but when I come to a stop light, there is VERY LITTLE left on the pedal before its to the floor....
any ideas? please don't tell me this is just the way it is!!
b
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I've
clean
come
I'd try replacing the combination valve..........
Doc

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Raybestos
have
the
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

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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

curious
no
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that.
fresh,
is
I
to
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rear.
when
hence
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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
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the
until
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.

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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
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no
the front flex hoses were brand new when I bought the truck 3 years ago....could it be that they've deteriorated already?
b
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Nah, not if they're that new............
Doc

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I've
clean
come
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
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"Hamilton Audio" wrote

And "many" are right.
Ian
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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

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