brakes won’t bleed

[bode winham]

i don’t know where else to post this so redirect me if this is the wrong place.

i’ve had super spongy brakes recently so i replaced the master cylinder and went to bleed the brakes. the front calipers bleed great but with the back drums i can only get a slow trickle out when i try to bleed. i replaced the wheel cylinders thinking maybe they were blocking the fluid but it didn’t make it bleed any easier.

i had ruled out the proportioning valve being the problem since there is still a small trickle but could it be the proportioning valve? i saw some threads on an abs delete, would that help? any suggestions would be great i wanna be able to drive my truck again

88 chevy c1500 silverado

 

[89obsSB]

Sounds like it could be a proportioning valve. Possibly collapsed rear soft line. Or if the master wasn’t bled properly there could be air in the rear valve for the master. Did you bench bleed it and cap the ports before putting it on?

 

[delta_p]

If you think it is the valve you can probably check it as follows and it is only meant to be what i would try.

put the parking brake off then turn the key on and wait a second, make sure the brake warning light goes off after a few seconds (park brake on and the light will stay on). if the warning light stays on, go unplug the brake warning light connector on the valve and then go an make sure the light went off. In this case it is the combination valve. Just to be sure, with the connector off get a multimeter with continuity and touch one probe to connector pin (on the valve not the harness) and the other to anything connected to ground, the side of the truck, a tube, whatever. if there is continuity then the brake warning spool has shifted over and could be blocking flow to the rear.

If you turn the key on and ect. as above and the light goes out, everything looks ok with the valve. But just to be sure take off the brake warning connector and hook up the multimeter to the connector pin like above and and then go and bleed the rear again. If you hear the continuity meter go off or beep then the spool is shifting while you are bleeding and you might need to gravity bleed a while or try pulling it through with a vacuum for a while instead of by pressure application through the brake pedal.

Sometimes you can reset the brake warning spool by applying brake pressure during bleeding and cracking open the opposite bleeder. Like if the spool has shifted towards the rear, cracking a front bleeder can make it shift back the other way.

 

[Schurkey]

Sounds like it could be a proportioning valve.

Possible. Not likely.

Possibly collapsed rear soft line.

Likely. I've had multiple OEM-rubber brake hoses on various vehicles that plugged up tighter than a bull's asss in fly season. NO amount of standing on the brake pedal would push fluid through those hoses. Brake pedal was high and firm (almost hard) and there was surprisingly little braking power.

It's not that the hose "collapses", the rubber swells and closes-off the fluid path.

At some point between "new" and "plugged", they'd merely be "restricted".

Other possibilities are that the metal brake tubing is dented-shut (or nearly shut) somewhere; and the RWAL isolation valve is stuck shut. RWAL has a bad reputation on this forum, but if that truck were in MY driveway, I'd (carefully) unhook the rear brake tube from the hose at the frame bracket, and see how much fluid was available during bleeding. Lots of fluid there almost certainly means a plugged hose.

Or if the master wasn’t bled properly there could be air in the rear valve for the master. Did you bench bleed it and cap the ports before putting it on?

Either the air would bleed out, or the air would make no difference in the fluid flow. Air in the system isn't blocking the flow of fluid. Trapped air could cause the pedal to be "super spongy", though.

If you think it is the valve you can probably check it as follows and it is only meant to be what i would try.

put the parking brake off then turn the key on and wait a second, make sure the brake warning light goes off after a few seconds (park brake on and the light will stay on). if the warning light stays on, go unplug the brake warning light connector on the valve and then go an make sure the light went off. In this case it is the combination valve. Just to be sure, with the connector off get a multimeter with continuity and touch one probe to connector pin (on the valve not the harness) and the other to anything connected to ground, the side of the truck, a tube, whatever. if there is continuity then the brake warning spool has shifted over and could be blocking flow to the rear.

I see this on multiple forums; and I don't know where it comes from.

The safety valve / spool doesn't block fluid flow.

MOST if not all GM safety valves are self-resetting. They're spring-loaded to the center position, they only go off-center and turn on the dash warning light when there's actual pressure difference between the two master cylinder hydraulic circuits.

I have dicked with FORD safety valves (twice) that had to be manually re-set. Ford's "better idea", I guess. Foookin' pain in the tuckus. Hateful job. However, even the Ford safety valves, when off-center, don't block fluid flow.

 

[Schurkey]

i’ve had super spongy brakes recently

The iron-body RWAL on an '88 should have it's own bleeder valve. Have you bled the RWAL?

The rear "Leading/Trailing shoe" brakes on C/K 1500s are notorious for being out of adjustment, which causes a low pedal. Be sure to manually adjust the rear brakes, assure that the rear adjusters WORK, and then use the park brake regularly to keep them in adjustment.

 

[delta_p]

In the combination valves I've seen, there's no springs on the pressure differential spool. And in cross sections you can clearly see there's an additional port on the rear side system to take away the function of the rear proportion when the front system loses pressure. I mean high loss, like full delta pressure against the spool.

I think the "springs and the doesn't restrict flow" on forums comes from the old individual brake warning device from the years of 19wayback. Ours are clearly not those valves, they are combination valves, they were designed to perform many different functions.

Hears a pic someone here took of a 96-99 combination valve guts. Here's a pic of the late 80's early 90's style too. The only spring is the one on the metering valve.

 

[Schurkey]

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The spring is on the safety switch, acting on the taper of the spool valve to center it.

Enlarged, you can see the outline of the safety switch spool moved entirely to the rear--it's got a bump on the back so that it CANNOT block the fluid passage.

Select scans from the AC-Delco service manual for brakes (yep, it does go back a few years.) Most of the safety-switch info is on the first page.

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[delta_p]

The flow of brake fluid is not restricted when the spool is all the way over there toward the rear, like if trying to bleed an empty line or blown out a rear line(?) It looks like it’ll move into primary path from the master. Restricted, not valved off closed(?)

Edit: I should know better than to do two things at once : ). The above was meant to be a question and not a statement.

 

[delta_p]

The valves shown in the service manual above are not the same valve as in the the trucks. The rear proportioning valve in the trucks is a serrated cup seal that allows flow under normal braking but seals off under hard braking (or loss of pressure like a blown line). And there is also bypass port such that when the front wheel pressure fails and the safety switch moves toward the front, the rear wheel bypass port is opened and allows rear pressure to bypass the proportion valve. the service manual doesn't show this bypass and the valve is different. So I am wondering what other design differences there are. Like if the safety switch spool can move into the master cylinder feed port and restrict flow and cause a dribble when trying to bleed the brakes.