Master cylinders are a relatively easy swap, if you have a pressure
bleeder.
Pump 'n hold, not so much. If that is how you bled your brakes after
you repaired them, you have air in the system.
Perhaps power bleeding might fix that if the master cylinder is good.
BUT, it likely is not, due to its location, it is exposed to rain water
leaking through the hatch cover, and has an affinity for absorbing water,
particularly if frequent fluid changes are not performed. Water and brake
fluid are not happy when mixed. Buy the highest quality USA made master
cylinder you can find, follow the bench bleeding procedures, and do your
due diligence on brake inspections. Brakes are nothing to screw around
with. It means your lives if they fail.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

On Tue, Apr 16, 2019, 7:45 AM Todd Sullivan via Gmclist <
gmclist@list.gmcnet.org> wrote:

> There are others here who are far more knowledgeable on brakes that I am
> sure will chime in soon. I am not sure that a failing master would give you
> intermittent issues. That sounds more like a booster. There is a check
> valve on the booster where the vacuum line attaches off the engine. I would
> check the hose from the motor to the booster for cracks/dryness/looseness.
> The check valve has a rubber grommet that seals it to the booster and the
> check valve itself could be faulty (it is basically a one way valve so I
> think you can test it with a suck n blow test) you replace the rubber brake
> hoses at the calipers/wheel cylinders?
>
> Sully
> Bellevue wa
>
> On Tue, Apr 16, 2019 at 6:35 AM Justin Brady via Gmclist gmclist@list.gmcnet.org> wrote:
>
>> Thanks Sully,
>> Any way to really tell?
>>
>> Basically every few presses it's soft, releasing and repressing usually
>> gets it to work OK but not 100%. I've been noticing them getting worse
>> lately
>> so I rebuilt the rear, bled, and changed pads in the front which helped
>> but I still felt it was a little off. Then on the way home from our last
>> trip
>> I started having to double pump the brakes to get them working so my
> logic
>> is saying it was the master leaking a bit all along and it's just now
>> getting bad enough to full leak past the piston.
>>
>> I hadn't really considered the booster. Hmm.
>> --
>> Justin Brady
>> http://www.thegmcrv.com/
>> 1976 Palm Beach 455
>>
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And when I posted that I was referring to a hand - held squeeze vacuum pump. Then I remembered, I have a perfectly good HVAC vacuum pump, what would be the argument against using it with a capture bottle? Should it not be the same as the pressure bleeder with 29.92 PSI on the master cylinder?

Physics gurus?

--johnny

Johnny Bridges wrote on Tue, 16 April 2019 14:07

And when I posted that I was referring to a hand - held squeeze vacuum pump. Then I remembered, I have a perfectly good HVAC vacuum pump, what would be the argument against using it with a capture bottle? Should it not be the same as the pressure bleeder with 29.92 PSI on the master cylinder?

Physics gurus?

--johnny

Johnny,

29.92 is Inches Hg.
14.7 PSI.
One place I worked used to pressure bleed at 30 psi right up until a guy shot brake fluid into his eyes.

I have been successfully vacuum bleeding for year. I like it because:
A - I have a little electric vacuum pump that makes it real easy.
B - It works for the newer and strange cars that have a plastic fluid reservoir.

It works even better if you wrap the bleed screws with teflon tape. That tape also keeps the bleed screws from corroding into the bodies....

Matt