Vacuum setup problem: pedal feels good, but braking is weak.

T

TommyV8

Member
Joined
Jul 7, 2004
After multiple Powermaster failures I ordered a remanufactured booster (9" double diaphragm) and master cylinder for an '87 Monte Carlo through Rock Auto. I paired them with a brake pedal from a naturally aspirated '83ish Regal. The brakes were bled properly after install, but the braking power is pretty weak. The pedal feel is ok, if a little soft. Sitting at a stop I can feel that the pedal is pretty much all the way down, and if I lift just a little the car starts to move. The pads are good, and the engine is making about 16" of vacuum. I've read that if the booster is bad then you'll get a hard pedal (like I got when the PM took a dump), but that is not the case here. I've also read that a good pedal feel but weak braking can be caused by a mismatch between the booster pin and the master cylinder (not enough pin length, too much pin hole). I ordered the booster and the cylinder for the same car. Any ideas? Thanks in advance.

Tom
 
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I would rebleed the brakes first as this is usually the source of soft braking on a changeover.
 
That's what I thought, so I took it to a shop to have them bled again. They said that they got all the air out, but still the problem persists. You're right though, the problem feels just like when you have air in the lines.
 
OK, they bled the brakes, BUT......did they bench bleed the M.C. ???
 
are the REAR brakes adjusted properly?
to much clearance and you WILL get a low pedal.
does car squat when braking, or does the nose "dive" when normal brakes area applied?
 
As above, I would rebleed and start with the master cylinder, then work from furthest to nearest. When you bleed are you getting good fluid flow from the wheel cylinders/calipers? Are you sure the m/c is good. I know charlief1 has had a problem with rebuilt m/c in the past.


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I can't tell if it's sqatting or diving. I'll check the rear brake adjustment, as well as have the cylinder bench bled and bleed the brakes again. Thanks for the help! :)
 
Slightly off topic, but is the MC master cylinder cast aluminum or cast iron?
 
All new master cylinders for g-body cars will be cast iron. Some rebuilt units can be cast iron or aluminum. Strait bore master cyliners where used from 1978 to 1981 (mostly cast iron and some aluminum) with vacuum assist. Step bore master cylinders where used from 1982 to 1988 on G-body cars (Stock/rebiult were aluminum, new are cast iron) with vacuum assist.

I believe the power master units used 1.0", strait bore, aluminum master cylinders. I don't think they make new ones in aluminum or cast iron. Rebuilt ones are hard to find, but an alternative master cylinder should be a rebuilt unit from a 1978 Buick Riviera with 4 wheel disk brakes. The 4 wheel disc Riviera has a 1.0", strait bore, aluminum master cylinder. New ones for this are most likely to be cast iron, rebuilt ones are aluminum. The reservior will most likely need to be changed out if using on a power master.

Strait bore and step bore master cylinders used different vacuum booster because of the size of the master cylinder housing.
 
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TommyV8 said:
After multiple Powermaster failures I ordered a remanufactured booster (9" double diaphragm) and master cylinder for an '87 Monte Carlo through Rock Auto. I paired them with a brake pedal from a naturally aspirated '83ish Regal. The brakes were bled properly after install, but the braking power is pretty weak. The pedal feel is ok, if a little soft. Sitting at a stop I can feel that the pedal is pretty much all the way down, and if I lift just a little the car starts to move. The pads are good, and the engine is making about 16" of vacuum. I've read that if the booster is bad then you'll get a hard pedal (like I got when the PM took a dump), but that is not the case here. I've also read that a good pedal feel but weak braking can be caused by a mismatch between the booster pin and the master cylinder (not enough pin length, too much pin hole). I ordered the booster and the cylinder for the same car. Any ideas? Thanks in advance.


Tom
Click to expand...
From 1982 to 1988, g-body used step bore master cylinders with LOW drag calipers. LOW drag calipers need extra fluid volume to proper operate which is regulated by a bypass valve which is built into the master cylinder. Because of this built in valve, they are harder to bench bleed and take longer to bench bleed. If bench bleed incorrectly, there may still be air in the master cylinder that is causing you issues.

Do an internet search on "low drag caliper" or "step bore master cylinder" or "how to bleed a step bore master cylinder" for more information.

Usually, rebuilt calipers are NON low drag calipers. New, bolt in, "metric" style calipers are NON low drag. NON low drag calipers can use both strait bore and step bore master cylinders, but stock (originals from the factory, low drag calipers) can only use a step bore master cylinder.
 
I had the same problem when I did my conversion as well. Turned out to be my check valve was not good and neither was my vacuum source. So I bought a check valve from Summit, made up a Russel line, Taped my intake spacer and hooked it all up. Brakes work excellent now. Better then I expected. See pics below.

Summit Check Valve



Russel Line


Hook up to the check valve


Hook up to the Motor
 
I guess I'll take off the master and make sure to bench bleed correctly, and then I can get a new check valve and replace that while I'm at it, cheap and easy. Thanks guys!:D
 
just a suggestion, i had the same issue, i did the ls1 front upgrade, and the car brakes sooo much better now, def recommend the upgrade. it may have been a bleeding issue, bc they bled the front again while the upgrade, or it may have been the oem braked with a vac setup was not ideal. not sure but if you can put bigger wheels out front, i def recomment the ls1 brakes. was night and day. i can actually drive around with some confidence that the car will stop when i need it o
 
Until recently I owned an LS1 Camaro, and that car did indeed stop WAY better than this GN ever has.
 
I woud have to guess that LS1 f-body came with larger rotors and larger tires allowing it to be better equipped. This equipment gets it stopping much better.

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Yes, it had larger discs in the front and smaller discs in the back, compared to the small discs in the front and drums in the back that our Regals come with. And the footprint for the Camaro was slightly larger, 245 wide tires.
 
Can u give me the part # to the check valve with an fitting on it or did u make. That looks good ttypewhite

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