Please help me on brake conversion!

Gnfanatic

Not-quite-so-new-guy
Mar 24, 2020
16
3
Hello everyone, I am losing my mind and need idea's. I have a 1986 Buick Grand National and the powermaster unit was going bad so I did a vacuum conversion.,, this is what I installed.
1990 Chevy caprice booster (No switch)
1987 Monte SS MC
brass proportioning valve from kirban for rear drums
Vacuum pedal

I bench bled the MC and I also checked to make sure the check valve on the booster holds vac which it does (The booster has its own dedicated 3/8 vac line from intake). After bleeding and bleeding and bleeding the system (Manually, mity vac and gravity) I still have a long travel pedal and the car barely stops. I cant even lock up the wheels. I do not know what is going on and cant figure out what to do. I went through a gallon of Dot3 bleeding these brakes. Please help!
 

CopperNick

Royal Smart Person
Supporting Member
Feb 20, 2018
2,247
113
Canada
Somewhere else on this site there is a thread about someone in a similar situation who was having issues with the proportioning valve, possibly due to the source. There is a tool available to lock the valve in the bypass position while you are bleeding the brakes. To get the valve to bypass, push on the rubber cap at the front of the valve which in turn pushes on the top of the valve spool underneath it. That shifts the spool to bypass and the cute little tool shown in the pictures in that thread gets screwed into the port on the top of the valve. The pin lines up with a groove in the spool and holds it in place until the bleeding is done at which time the tool is unscrewed and the port cap replaced. First time you drive the vehicle and use the brakes the prop valve resets itself and "Bob's your uncle'.



Nick
 

Gnfanatic

Not-quite-so-new-guy
Thread starter
Mar 24, 2020
16
3
Thanks Nick, I am surprised Kirbans did not include that tool. I am also suspicous of the pushrod length inside the booster, possibly to short for the mc.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
4,992
113
Upstate NY
I’m not understanding the ‘Caprice booster - no switch’ - what does konswitch mean? And why did you use a Caprice booster rather a G-body booster? Is it a single or double diaphragm?

And what’s is a power master unit?
 

Gnfanatic

Not-quite-so-new-guy
Thread starter
Mar 24, 2020
16
3
I’m not understanding the ‘Caprice booster - no switch’ - what does konswitch mean? And why did you use a Caprice booster rather a G-body booster? Is it a single or double diaphragm?

And what’s is a power master unit?
The 90 caprice has a 11.75" dual diaphram booster. one has a sesnor on opposite side of check valve, I got the one with just a check valve. It is larger then the Gbody booster, thats the reason. Grand Nationals came with a electric braking system called Powermaster unit.
 

CopperNick

Royal Smart Person
Supporting Member
Feb 20, 2018
2,247
113
Canada
The absence of the tool in your kit is due to the fact that it is apparently not cheap to buy. The ones I mentioned from that other thread were apparently worth far more than a 2-4 case of beer or a half tank of gas.

As for choice of brake booster, that double diaphragm unit is meant to assist with braking on a B body car which is heavier than your GN. You might be correct in your theory that the actuator piston for it is mismatched for what you need.


Nick
 

Gnfanatic

Not-quite-so-new-guy
Thread starter
Mar 24, 2020
16
3
Nick, not sure what you mean about the cost of the tool. I ordered it and is coming in this week. I checked the pushrod and it was to short, fixed it and I still have a very very soft pedal. It is awesome when the car isnt on but once started the pedal goes down big time
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
4,992
113
Upstate NY
The 90 caprice has a 11.75" dual diaphram booster. one has a sesnor on opposite side of check valve, I got the one with just a check valve. It is larger then the Gbody booster, thats the reason. Grand Nationals came with a electric braking system called Powermaster unit.
A stock G-body dual diaphragm booster was standard equipment for HD version. And they work great.

Anyway, if you’re not sure if the rod in the booster is long enough, then resolve that 1st.
 

Gnfanatic

Not-quite-so-new-guy
Thread starter
Mar 24, 2020
16
3
A stock G-body dual diaphragm booster was standard equipment for HD version. And they work great.

Anyway, if you’re not sure if the rod in the booster is long enough, then resolve that 1st.
Rod length has been resolved but did not help. I am getting a gbody booster tomorrow to try
 

565bbchevy

Geezer
Aug 8, 2011
9,215
113
Michigan
I still have my original 9" dual diaphragm booster on my Monte which also gives me the clearance for the BBC and it works perfect with a vacuum canister and a pulley driven vacuum pump that pulls a constant 28 lbs.
IMG_1384.JPG
 
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