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JJ Burgess

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ok. So I was getting tired of the spongy pedal on my 88 C1500. Decided to do the RWAL delete and new stainless hoses. After bleeding the brakes the pedal was even softer than before. So I went and got a NBS master cylinder. Bleed the brakes again and it just as bad if not worse. I can’t figure it out. I noticed yesterday that on the proportioning valve there is a little bit of fluid coming out of where the rear plunger pin comes out. Could that be my problem? Also. I have a pneumatic bleeder and when I do the rear brakes I’m never really getting just straight fluid. I’m not sure if that’s just how drums are or if I’m getting that much air in the system. With the truck off I can get a very solid pedal with no creep. But with the truck on the pedal goes to the stop. I took this picture from someone online but this is the exact setup I have. Where I circled it is where the little bit of fluid is coming from. Please help!
 
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RichLo

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I see you bench-bled the MC before installing it which is good. But you should always see straight fluid when your finished bleeding any brake system. Keep going until there's no more bubbles then go another couple cycles. And always start with the Pass rear, then Drivers rear, then Pass front, and lastly Drivers front.
 

JJ Burgess

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That’s exactly how I did it. But no matter how much I bleed I still have bubbles in the rear. I get a firm pedal and it does creep as if there was a leak but I just can’t get those bubbles out. I’m gonna try more today. Could air be drawing in from that proportioning valve?
 

DerekTheGreat

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Well, I don't see any fluid seeping from that area on my '89. I'd say try another prop valve, no leaks in the braking system are acceptable IMO.
 

RawbDidIt

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I had a similar problem with my truck, minus the outpouring valve. Mine has three JB6 brake code (11" rears) and it finally cleared up when I fixed a vacuum leak in the PCV line. Check your vacuum, I thought mine was good, because it wasn't making that telltale whistle, went to pull the PCV line to change my fuel pressure regulator, and the boot was split.

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JJ Burgess

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I’ll have to check that out. So just check the hose going to the booster? I figure that would make it act like you didn’t have power brakes rather than make the pedal go soft. Today I went through all the lines. Bled them for the 6th time and still have a soft pedal. It gets hard for a little bit. Then let it set 5 mins and when I step on the brake with the truck off it goes to the floor. Could the new master cylinder be bad?
 

RawbDidIt

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I’ll have to check that out. So just check the hose going to the booster? I figure that would make it act like you didn’t have power brakes rather than make the pedal go soft. Today I went through all the lines. Bled them for the 6th time and still have a soft pedal. It gets hard for a little bit. Then let it set 5 mins and when I step on the brake with the truck off it goes to the floor. Could the new master cylinder be bad?
Check all vacuum lines. There's one to the power booster, another to the PCV (where I had my leak) and another on the passenger side that goes to a solenoid for something for other (sorry, can't remember what the component is) it acted exactly like you said, solid when off, way too much pedal travel before engagement, and very weak brakes when at the floor.

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evilunclegrimace

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A vacuum leak in the intake system will NOT affect the Hydraulic brakes system as far air in the system. The combo valve CAN be sucking air in especially if you have fluid leaking out.

Spray the Combo valve down with brake cleaner and dry it off completely with compressed air, then pump the brake pedal up until you get a hard pedal and hold firm pressure on the pedal and see if it bleeds down. Check the Combo valve for being wet also carefully check ALL of the brake lines AND connection's for seepage, ANY fluid leaks mean that air can/is getting into the system.

EDIT to ADD. The Vacuum system can pull brake fluid out of the Master cylinder IF the rear seal on the Master cylinder has failed.
 
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