Power master pressure switch

screamingbuick

New Member
Joined
Jul 29, 2010
Hey guys, its been a while, but the cars back on the road after sitting for almost 7 years. My son is now driving my 87 GN. Does the electric motor below the Power master ever turn off, i would think it would after its reached its required pressure? There doesn't appear to be any more leaks, I've gone through and replaced a bunch of lines after it sat for so long. Is the switch on top replaceable and if so where can i get one?

Thanks for any help,

Bob
 
If the electric motor is running too much or constantly, DO NOT DRIVE ANY MORE!!!! You will burn it up. It could be the pressure switch or the accumulator ball. They are both extremely easy to replace.

Key off!! Pump brake pedal at least 10 good pump to relieve pressure. Pedal will get hard. Then just unscrew the part to replace.

NOS4GN/Highway stars has them, as does Kirbans.
 
Just curious.
Let's say you've replaced the switch and accumulator ball with known good units, and also the PM Motor is known good.
What else could cause the PM motor to run non-stop?
 
Just curious.
Let's say you've replaced the switch and accumulator ball with known good units, and also the PM Motor is known good.
What else could cause the PM motor to run non-stop?


Possibly a bad check valve.

One of the obvious signs of a bad check valve is: Turn key on and sit around and wait. Does the motor cycle/run on its own after some time, then do it again after some time passes, with NO interaction by the owner?
 
Possibly a bad check valve.

One of the obvious signs of a bad check valve is: Turn key on and sit around and wait. Does the motor cycle/run on its own after some time, then do it again after some time passes, with NO interaction by the owner?

Where is the check valve located?

Motor runs as soon as soon the key is turned on and won't shut itself off. Acts like it does not ever build enough pressure to shut the motor off.
Switch, motor and accumulator are all known good units.

What about wiring?
 
Where is the check valve located?

Motor runs as soon as soon the key is turned on and won't shut itself off. Acts like it does not ever build enough pressure to shut the motor off.
Switch, motor and accumulator are all known good units.

What about wiring?

Howdy.
Any one?:redface:
 
pm.jpg
 
Thanks. That diagram definitely helps, but...I still don't know where the "check ball" is. Didn't see it on the diagram list.:rolleyes:

...and yes...I'm stupid (according to my wife)...so don't ask.:D
 
I am not sure what TurboDave is referring to, but I posted the diagram in hopes he can use it to pinpoint it for you. I was thinking 36. :)
 
Thanks. That diagram definitely helps, but...I still don't know where the "check ball" is. Didn't see it on the diagram list.:rolleyes:

...and yes...I'm stupid (according to my wife)...so don't ask.:D

The check valve is parts 36, 37, 38.... Kinda hard to get pulled out.

I'm sure you realize before unhooking anything that you must releave the system pressure....Key off, 10 pedal pumps.

Did you replace the accumulator and pressure switch? Is that the reason you're convinced they're OK?

I'm not entirely sure this will be your issue. Sounds like you may have a massive internal leak.
 
The check valve is parts 36, 37, 38.... Kinda hard to get pulled out.

I'm sure you realize before unhooking anything that you must releave the system pressure....Key off, 10 pedal pumps.

Did you replace the accumulator and pressure switch? Is that the reason you're convinced they're OK?

I'm not entirely sure this will be your issue. Sounds like you may have a massive internal leak.

Howdy.
Yup...tried a couple different switches, accumulators, a different master cylinder, a couple different motors...basically...the pumps are the parts that we suspect are bad...

Finally today we located a good used PM and swapped the whole PM in. Works like a charm.:D

Gonna replace my proportioning valve too. I still have one of the original ones that apparently were supposed to be replaced via recall.
 
Well, that was not fun, got a new accumulator from Kirbans, and that isn't the issue, I guess the switch is next, maybe I should have done the cheap fix first, ughhh.
 
Hey no problem, I'm thinking about going to vacuum brakes now, just don't want the headaches any more. Makes for a much more simple system.
 
I don't blame you. When you are looking for a remanufactured complete unit, it's hard to stomach $600+. I don't think I'd even waste the time and money to find a used replacement since it could last a month for all you know.
 
Well, that was not fun, got a new accumulator from Kirbans, and that isn't the issue, I guess the switch is next, maybe I should have done the cheap fix first, ughhh.

I still have that switch if you need it. Also wondering if you might have an internal leak. My understanding is the unit has to reach XX PSI for the motor to turn off. The switch on top turns on/off the motor. Perhaps you could pull the switch (with the powermaster pressure pumped down via 10 pedal pumps til hard) and check it with an ohm meter to ensure it's not stuck closed?
 
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