brake booster - Ford Mustang Forum
 
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post #1 of 4 (permalink) Old 1 Week Ago Thread Starter
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Charlie DuBois's Avatar
 
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brake booster

1967 Mustang 289 HP. 19" vacuum.

I have done a disc brake conversion for the front and new drum brakes on the rear with new brake cylinders. Proportioning valve for the rear as part of the conversion. All new brake lines and flex hoses. Master cylinder came with the kit. No leaks and pedal remains firm when holding. 10# inline check valve to the rear brakes. Good flow of brake fluid when bleeding. Braking was very poor before changing rear brakes and installing the conversion.

I have poor braking and with a gauge I get 300# at the cylinders and disc bleeder valves. I know that is not correct. I did not buy the conversion from CJ Pony but I called them and they said the master cylinder was bad. Ordered one from them and no improvement. I used a tool to set the master cylinder to booster and verified that the setting was correct by taking measurements. The master cylinders ports are fully open and I see movement after a slight push on the brake.

Distribution block may be original and I know that the piston is centered and does not prevent flow to either front or back brakes. I have verified the warning light works because I removed it and ground it out by pushing it on bare metal.

Been searching the internet and found systems of how to detect a bad booster. Per the previous owner the brake booster was a retrofit and relatively new so it was reused.. No vacuum leaks anywhere. After shutting down the engine I waited TEN minutes and removed the check valve and there was a swoosh which tells me that it was holding a vacuum. Check valve checked and it is holding. Same swoosh after several removals of the check valve. There is no information on the booster but after searching it appears to be a Midland. 7" diameter.

Today I hear a hissing noise when stepping on pedal and engine RPM's drop. No pedal movement after dumping the residual air or when restarting the engine.

I do believe the booster is bad but, if so, why does the booster hold air after shutting engine down?

I have searched the internet but have been unable to find any info. as to whether a good booster would increase pressure at the discs and drums. If it does that may resolve the issue of poor braking.

Opinions?

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post #2 of 4 (permalink) Old 1 Week Ago
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I'm by no means a brake expert, but it sure sounds to me like the booster is leaking when you press the brake pedal. To verify, hook up a vacuum gauge to any vacuum line and put it where you can see it when you press the pedal (or get someone to watch it). I suspect it will show a loss of vacuum.

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post #3 of 4 (permalink) Old 6 Days Ago
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73 351Q's Avatar
 
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with engine off, pump the pedal until pedal is hard [no vacuum in booster] with your foot on the pedal, start the engine. if pedal drops, booster is working, if it doesn't drop, booster is either bad or getting no vacuum. put a vacuum gauge on hose at the booster, if you have good vacuum, i'd replace the booster
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post #4 of 4 (permalink) Old 6 Hours Ago
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Just a couple things to help out: Mushy brakes = air in the lines, or hoses flexing/leaking.

Hard brakes (excessive pedal pressure required to stop the car) can be a problem with master cylinder diameter, or booster problem. Bigger MC diameter = more pressure required to stop, and less pedal travel. Smaller = stops easier, but requires more pedal travel.

Is the 'loop' on the end of the master cylinder rod straight out, or curved down? On the earlier cars, the loop hangs 'down' to achieve proper angle with the brake pedal. Using a 'straight out' loop puts things in a bind, so you can never get proper braking, and can actually damage things. Another issue that can crop up is trying to use a manual brake pedal with a power booster setup. All of these angles and relationships should be taken into account for the conversion kit you bought, hopefully.

The bleeds for brakes are always at the top. This is so air bubbles (which will obviously float in brake fluid) can get out. Sometimes instructions for installation don't make this clear, and things get installed upside down, trapping a nice big bubble of air in the piston chamber(s), no matter how much bleeding you do. Of the brake fluid. If you're bleeding personally, you are probably doing it wrong.

Brake pads matter. A lot of times, even a good set of brakes get sold with cheap, crummy pads that have a low friction coefficient. A set of good pads will make a tremendous difference. Avoid full on race/metallic pads because they don't work very well until they get pretty hot, and that should not normally occur while tooting around town.


Hope this helps!

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