1997 Chevy K1500 master cylinder question

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HerpDerp1919

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Hey all, so I've been trying to fix an extremely soft brake pedal issue. I've bled all my brakes, just replaced my rear drums and shoes as they were extremely worn and figured that would solve the issue. It did not. I'm at a loss of what could be causing the issue. So I'm looking at my MC again, I already replaced it once, bench bled and im still having a soft pedal.
So my question is, when I bench bleed, should the cylinder be extremely hard to push in? All the videos I watch on how to bench bleed, these guys make it look like it should go fairly easy. I'm not the strongest guy out there, but I also eat my wheaties and this thing is a bear to push in to bleed. It also shoots fluid up out of the reservoir, similar to a stream if you were to bleed a caliper. Is that normal or did I get a faulty unit from O'reilly's?
 

grey wolf

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I think the soft brake pedal is a pretty common issue with our trucks. I've also got the same problem, but it's something that seems to have a few different approaches to fix. I've more or less just grown used to it now. I've also heard that you need a scan tool to properly bleed the brakes if you have ABS? Maybe someone can clear that up...

As for the master cylinder bench bleed, I've never done one for our trucks, but as a tech I've done my share of MCs over the years. I have a kit that redirects the fluid from the ports back into the reservoir when you are bench bleeding it. It's not necessary, but it is convenient and a lot less messy. You can probably find it at an auto parts store or online somewhere. Mine's made by a company called "Thexton". You can also just use your fingers to plug the ports when you release the piston back to its resting position, to prevent air from getting sucked back into the cylinder. Also, that fluid shooting up from the reservoir is normal.
 

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RWAL does not need a scan tool. Early, cast-iron bodied RWAL had their own bleeder screw. Later aluminum-bodied RWAL don't.

For the newer vehicles that have 3-channel ABS, the scan tool is required--see .pdf attached. Some folks "get by" with stomping the brakes on gravel; but to do this right you have to make sure that all three channels of ABS actually engage, not just some of them. And then you have to drive back to the shop to bleed the brakes in the normal way. Moving air out of the ABS does not get the air out of the brake hydraulic system. That's done by bleeding at the wheel cylinders/calipers after activating the ABS.

Are you trying to bench-bleed that master cylinder with the plastic plugs still installed in the outlet ports?

Are the rear brakes properly adjusted? Are these the hateful, terrible 254mm (10-inch) drums?
 

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HerpDerp1919

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Are you trying to bench-bleed that master cylinder with the plastic plugs still installed in the outlet ports?

Are the rear brakes properly adjusted? Are these the hateful, terrible 254mm (10-inch) drums?
Apologies for a late reply, life got busy for a couple weeks and forgot about this post.
When I bought my MC I also bought a cheap bench bleed kit from Oreillys that came with adapters and some small lines to send fluid back into the reservoir and proceeded to bench bleed.

So on all my other trucks (I've owned a lot of 2500s, not so many 1500s) I could always adjust the rear brakes through the backing plate, on this 1500 they are the auto adjusting ones that I've read are supposed to take up the slack when you hit the brakes in reverse. They also are not the 10" but the 11.15" drums.
 

Jesse_Wenting

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So I'm kinda going through some ABS woes myself right now.

I replaced my entire brake system around last July and did a hydroboost swap. Bleed everything and had a really nice pedal feel until come winter and eventually my ABS kicked in, and ever since then if I'm at a red light for a few seconds the pedal will slowly sink to the floor and the dash brake light will come on. The truck stays in place, but it also takes a few seconds for the pedal to come back up if I don't pull it with my feet.

Any chance you're experiencing something similar OP?
 

Schurkey

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if I'm at a red light for a few seconds the pedal will slowly sink to the floor and the dash brake light will come on.
Sounds like a failed master cylinder. The dash warning light is almost certainly being lit by the "safety switch" between the front and rear hydraulic systems. Unequal pressure between the two systems will move the safety switch plunger, which makes the light glow.

it also takes a few seconds for the pedal to come back up if I don't pull it with my feet.
That sounds like a failed booster--but--I'm no expert on Hydroboost. Someone else needs to offer opinion(s).

I'm not sure how either of those could be the ABS unit. I think it would have to be two separate failures. If there were a leaking solenoid valve for one of the accumulators, you'd maybe lose fluid volume into the accumulator which would reduce pressure in one hydraulic circuit. That might trigger the warning light. But a different solenoid valve (an isolation valve) would have to be stuck shut to keep the pedal from returning. A simultaneous failure of two ABS valves seems...unlikely.

But then, so does a simultaneous failure of both the master cylinder and the booster.

"More diagnosis needed."
 

Jesse_Wenting

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Sounds like a failed master cylinder. The dash warning light is almost certainly being lit by the "safety switch" between the front and rear hydraulic systems. Unequal pressure between the two systems will move the safety switch plunger, which makes the light glow.
I got the hydro-boost off a 2000 parts truck. Guy said the MC was less than a year old.

When the dash lights comes on, theres a little "click" in the pedal. It happens just as the pedal reaches the end of its travel
 

HerpDerp1919

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So no update on ABS module yet because no mechanic around me has a scanner that is capable of doing it except one and they want $300 to perform the procedure.
However when I had a mechanic come out who thought his scanner could, he re-bled my brakes and I learned I've apparently been bleeding brakes wrong my entire life. After he re-bled my brakes the pedal got a little more stiff but it is still very spongy. So still on the hunt for a reasonably priced mechanic to do the ABS bleed.
 
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