Proportioning / combo valve failure?

[dremu]

Anybody ever have a stock combo valve go bad, and if so, how would I tell?

Truck is a stock C30. Brakes have never been great, but recently they got mushy and require tremendous pedal travel to stop the vehicle.

The master and booster were replaced 'bout two years ago.

I've just replaced the front calipers with rebuilds, and bled the thing until I'm blue in the ... well, it's bled. I've done bleeds before, using my home-made garden sprayer pressure job, and have had good luck with it, so I think the bleed isn't the issue. This thing being a crewcab, the brake lines are looooooong, and I've tried to be patient and make sure no bubbles come out.

One thing to note is that the rubber nipple/button on the combo valve seems to be permanently pushed in, or at least it won't pop in and out the way the one on my K5's do. (They're all 4WD, so maybe the valve is different? Looks the same, from what I can see upside down and sideways.)

Anyway, any takers on whether I'm an idiot and need to bleed the thing more, or whether the combo valve has a particular failure mode or what ... I'm all ears

-- A

 

[13lazer]

i dont know anything about your proportioning valve, but you might want to check your wheel cylinders in the rear drums. there are rubber seals in there that can go bad, and theres a tool to hone the walls just like the one for engine cylinders. Or you could buy new ones they are cheap.

 

[dremu]

i dont know anything about your proportioning valve, but you might want to check your wheel cylinders in the rear drums. there are rubber seals in there that can go bad, and theres a tool to hone the walls just like the one for engine cylinders. Or you could buy new ones they are cheap.

Ah yeah, forgot to mention ... gonna be switching out the rears for the 13x3.5" brakes in the next coupla days (got the 2.5"s now) so I'll be dang sure to switch out the wheel cylinders. Thought I replaced them two years ago with the master, but in retrospect, I don't think I did.

Aargh. This is the first truck to outsmart me on brakes.

Oh, and thanks!

-- A

 

[dremu]

Well, with a different set of wheel cylinders on the rear, the damn thing is still soft & spongy after I pressure bled the h3!! out of it ... so the only thing left is the combo valve. Nobody's ever had one fail?

-- A

 

[73k10sub]

I had one fail on a '73 Bel Air about 20 years ago. I do not know how I arrived at that being the problem(or who might have helped get to that conclusion). It locked the front brakes - I could not untrack it!
I put on another one, bled everything and was good to go. LOL!

 

[dremu]

Well, I have a spare ... prolly not right for the PU, as it's from a K5, but it's there, so I may put it on (aargh, I need more elbows!) and bleed, bleed, bleed...

repeat after me: I love hydraulics, I love hydraulics...

Thanks!

-- A

 

[bigbadchev84]

i just went throught the same issue on my brakes and i probably beld 5 gallons of fluid it seemed, i even went and built the garden sprayer setup, then it finally hit me, i was having probs with the hydro clutch in one truck and found you gotta hold the bleed screw at the top most position to get all the air out, well since i had installed new calipers and pads i removed the calipers and stuck a hose on the bleeder screw and twisted the caliper until the screw was up, then opened it and found there to be quite a bit of air still left in there even after the pressure bleed. try that and see what happens, i bet you will be suprised to find more air still. btw my brakes will now put you through the windshield

 

[dremu]

Hmm. The bleed screw on the wheel cylinders in back is straight up ... but intersting, it's kinda sideways on the calipers up front. I'll give that a roll ... those JB8 Bendix calipers are not fun to remove, but it's doable.

Thanks!

-- A

 

[bigbadchev84]

any luck?

 

[dremu]

any luck?

The calipers, actually, are positioned such that the bleeder screw is indeed at the top. I swapped combo valves with a spare I had, and now I am quite sure I have air in the lines =))

Rain came in, so prolly tomorrow I'll try and PATIENTLY bleed the system, see how it goes.

-- A

 

[boz42]

i have one truck & 2 k5's that deleting the combo valve fixed the mushy brake pedal problem. i don't know if it was cause there was air trapped in it or if it was broke/wore out.

i just staight plum the frt & put an adjustable prop on the rear