[Swishedb]

Hi, I have a 65 Chevelle and I replaced the master cylinder and booster with a chrome version.

The new one is 9 inches and has the flat bottom so it's not as long as the previous one.

Ever since install, the brake pedal is extremely hard to push, and I don't stop when I want, it has a longer stopping distance.

The mechanic said I needed a new proportioning valve but if it wasn't a problem before the install then I don't know why it is now.

Any ideas about what's causing this?

 

[Kevin McCaffrey]

It's possible your Booster is leaking vacuum. When the engine is running, put your ear under the dash and listen where the brake pedal shaft goes through the firewall. If you hear air rushing in, there is an internal vacuum leak in your brake booster. During normal operation, when your engine is running, and your foot is off the brake, vacuum is on both sides of the brake booster diaphragm. When you press the pedal, the shaft pushes on a valve which vents one side of the diaphragm to atmospheric pressure. This difference in pressure is the pressure-assist which makes power brakes easier to push. If that valve is leaking, you effectively have manual brakes.

 

[72mojave]

What measures 9 inches? The booster or the master? A bad check valve could cause this problem too. Even a new check valve can be bad (I know firsthand). Did you change the size of your booster from 11 inches to 9 inches? A 9 inch booster will provide less boost than an 11 inch and could be your problem. What master did you buy? Is it the same bore as your old master? There are a lot of variations in boosters and masters, you can't assume all work the same.

 

[Swishedb]

What measures 9 inches? The booster or the master? A bad check valve could cause this problem too. Even a new check valve can be bad (I know firsthand). Did you change the size of your booster from 11 inches to 9 inches? A 9 inch booster will provide less boost than an 11 inch and could be your problem. What master did you buy? Is it the same bore as your old master? There are a lot of variations in boosters and masters, you can't assume all work the same.

Hi, the booster is 9 inches and the master is 1 1/8 bore. The previous booster was the same diameter as the one now but there isn't as much depth due to the flat bottom, and the rounded one it replaced.

I still have the old booster but I'm not able to find any serial numbers to properly identify it. Only the bracket mounting it to the firewall had one: BR-1741-R.

 

[Swishedb]

It's possible your Booster is leaking vacuum. When the engine is running, put your ear under the dash and listen where the brake pedal shaft goes through the firewall. If you hear air rushing in, there is an internal vacuum leak in your brake booster. During normal operation, when your engine is running, and your foot is off the brake, vacuum is on both sides of the brake booster diaphragm. When you press the pedal, the shaft pushes on a valve which vents one side of the diaphragm to atmospheric pressure. This difference in pressure is the pressure-assist which makes power brakes easier to push. If that valve is leaking, you effectively have manual brakes.

Thank you, I'll check that out soon and see if it's the issue.

 

[Swishedb]

What measures 9 inches? The booster or the master? A bad check valve could cause this problem too. Even a new check valve can be bad (I know firsthand). Did you change the size of your booster from 11 inches to 9 inches? A 9 inch booster will provide less boost than an 11 inch and could be your problem. What master did you buy? Is it the same bore as your old master? There are a lot of variations in boosters and masters, you can't assume all work the same.

Here's the old and new booster.

What measures 9 inches? The booster or the master? A bad check valve could cause this problem too. Even a new check valve can be bad (I know firsthand). Did you change the size of your booster from 11 inches to 9 inches? A 9 inch booster will provide less boost than an 11 inch and could be your problem. What master did you buy? Is it the same bore as your old master? There are a lot of variations in boosters and masters, you can't assume all work the same.

 

[72mojave]

Hi, the booster is 9 inches and the master is 1 1/8 bore. The previous booster was the same diameter as the one now but there isn't as much depth due to the flat bottom, and the rounded one it replaced.

When you say flat bottom do you mean like this, with the horizontal ridge top and bottom? I haven't seen a booster where the bottom is cut off as the diaphragm inside is round. If you got the same size booster and master as the previous, I'd be looking at a leaky booster or bad check valve for starters. I don't trust repro boosters anymore. I bought my last from a restorer, Steve Gregori, brakeboosters.com.

 

[72mojave]

Here's the old and new booster.

Looks to me like you went from a 9" dual diaphragm to a 9" single diaphragm booster, in which case you now have less boost.

 

[72mojave]

This is a 9" dual diaphragm booster, looks like your old one.

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Comparison of psi for different size / diaphragm boosters:

https://www.classicperform.com/PDFs/BoosterBrakePressureChart.pdf

 

[lucifershammer]

Looks to me like you went from a 9" dual diaphragm to a 9" single diaphragm booster, in which case you now have less boost.

Yes the original was a dual 9”, the op has gone to a single 9” now. I had huge issues with that new size booster he has now and went to a dual 9” booster to resolve it. I would suggest going back to a dual 9”.

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[Swishedb]

Looks to me like you went from a 9" dual diaphragm to a 9" single diaphragm booster, in which case you now have less boost.

Yea, I messed up. I'll put the old one back on. To make matters worse, I just got on it and it tried to stall, limped it into my driveway and can't get it to start. The only thing I've done is adjust the Baer proportioning valve knob to see if it would make an impact.

 

[Swishedb]

Yes the original was a dual 9”, the op has gone to a single 9” now. I had huge issues with that new size booster he has now and went to a dual 9” booster to resolve it. I would suggest going back to a dual 9”.

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I'll go back to the original - any idea why the car kept stalling and now it won't start?

I increased the prop valve knob to max just to see if it would have any effect, and it's not starting since then.

 

[lucifershammer]

I'll go back to the original - any idea why the car kept stalling and now it won't start?

I increased the prop valve knob to max just to see if it would have any effect, and it's not starting since then.

Make sure all the vacuum lines are connected properly and no hoses are broken. Might be a vacuum leak issue that happened when you were swapping parts.