Brake booster question

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DusterDaddy

DusterDaddy

sledgehammer mechanic
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I bought a new booster/master cylinder for my Duster.

I took the master off the booster to paint the booster.

I put a vacuum gauge on the car today and had almost no vacuum on the small nipple at the booster. I pulled the fitting out of the booster and there was no difference in vacuum. I put my finger over the inside of the booster fitting and the vacuum went up to the normal range on the gauge.

So I'm thinking the booster can't build any vacuum at all. I must have assembled the master and it's gasket/spacers wrong that keeps it from sealing, or the push rod from the booster to the master cylinder is in there wrong.

Any thoughts?
 
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The master cylinder is not under vacuum. Right? The vacuum leak must be internal to the booster. Send it back for a replacement. Since it's defective, the seller should issue a shipping label to you for the return.
 
If a Mopar booster (photo would help), there are vacuum seals on both the input (sliding O-ring) and output (rubber bellows) sides. That is even true in 1990's cars (Breeze, Intrepid). There are thin pancake style boosters which seal vacuum on the output side via a rubber seal at the MC. My 1980's M-B and (I think) Mopar minivans are like that. Allows a thinner booster, but any brake fluid leaks get sucked into the booster. I agree that your booster sounds defective. You usually hear the air hissing in, most often from the cabin side where the brake rod goes in. Your engine would also idle faster from the increased air flow, except in modern engines where the IAC valve compensates to maintain rpm.
 
BillGrissom said:
If a Mopar booster (photo would help), there are vacuum seals on both the input (sliding O-ring) and output (rubber bellows) sides. That is even true in 1990's cars (Breeze, Intrepid). There are thin pancake style boosters which seal vacuum on the output side via a rubber seal at the MC. My 1980's M-B and (I think) Mopar minivans are like that. Allows a thinner booster, but any brake fluid leaks get sucked into the booster. I agree that your booster sounds defective. You usually hear the air hissing in, most often from the cabin side where the brake rod goes in. Your engine would also idle faster from the increased air flow, except in modern engines where the IAC valve compensates to maintain rpm.
Click to expand...

It's a stock remanufactured unit for a 74 Duster. I'll look for a closeup photo later today.
IMG_7547.JPG
 
This did not get used on the booster/master cylinder install.
I'm sure this is a big part of my problem
IMG_7560.JPG
 
Heehee!
Yeah put the bit in there before looking for a new booster. You will have to readjust your pushrod length, I'm pretty sure.
Also here's a test; with the engine running,clamp off the vacuum hose to the booster and note the engine rpm. Go step on the brake pedal about 4 times to exhaust the vacuum supply.Now check the tach,nothing should have changed.Next back under the hood remove the clamp. The engine rpm should immediately drop about 50 rpm, then recover back to the noted rpm. This would be normal.
Then shut the engine off. Pump the pedal again to exhaust the vacuum supply. Push the pedal down with about 50 pounds of force, and start the engine. The pedal should drop under your foot; that is normal.
If your booster works like this, and you cannot hear a vacuum leak under the dash, then it is ok.
I have to wonder how your brakes work tho, cuz without that bit in there it should not hsave been possible to shorten the pushrod enough for the compensating port to work.And that means the control valve inside the booster has never been centered, so in effect, the brakes would always have been applied. This would have made the brakes quite sensitive.
Another possibility is with the pushrod not adjusted, the first time you used a lot of pedal, the control valve woulda broke, and the engine would be continually sucking air under the dash.Of course with the broken control valve, he booster would no longer work.
 
AJ/FormS said:
Heehee!
Yeah put the bit in there before looking for a new booster. You will have to readjust your pushrod length, I'm pretty sure.
Also here's a test; with the engine running,clamp off the vacuum hose to the booster and note the engine rpm. Go step on the brake pedal about 4 times to exhaust the vacuum supply.Now check the tach,nothing should have changed.Next back under the hood remove the clamp. The engine rpm should immediately drop about 50 rpm, then recover back to the noted rpm. This would be normal.
Then shut the engine off. Pump the pedal again to exhaust the vacuum supply. Push the pedal down with about 50 pounds of force, and start the engine. The pedal should drop under your foot; that is normal.
If your booster works like this, and you cannot hear a vacuum leak under the dash, then it is ok.
I have to wonder how your brakes work tho, cuz without that bit in there it should not hsave been possible to shorten the pushrod enough for the compensating port to work.And that means the control valve inside the booster has never been centered, so in effect, the brakes would always have been applied. This would have made the brakes quite sensitive.
Another possibility is with the pushrod not adjusted, the first time you used a lot of pedal, the control valve woulda broke, and the engine would be continually sucking air under the dash.Of course with the broken control valve, he booster would no longer work.
Click to expand...

Well, that all makes sense.
2 things. When bleeding the brakes after the new master was installed ( by a mechanic I might add! ) there was a point when I as the pumper felt and heard a bit of a snap in the pedal or assembly. Next, the right front and right rear brakes have hung up completely at times during test drives around the neighborhood. The right front happened so many times I replaced the caliper and the hose. So the pushrod being too long must be always applying the brakes to a certain degree.
I do have the OEM style brakelines with the coils near the master cylinder so I can slide the master off the studs of the booster. I'll have at that later today.
 
You have the Midland-Ross booster (V-clamp). I took one apart from a 74 Dart slant. It had the bellows seal around the output shaft, so shouldn't need a vacuum seal between booster and MC. However, that missing gasket looks like it is intended to seal vacuum. The similar part in my booster was hard nylon, with a lower drain slot. Thus, yours appears to be a different design. Perhaps it changed mid-year in 1974. Anyway, when you remove the MC, you should be able to tell if there is an internal bellows between the output hole and the vacuum hose port. If open, then you certainly need a vacuum seal between booster and MC.
 
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