brake booster - Ford Mustang Forum
LinkBack Thread Tools
post #1 of 4 (permalink) Old 1 Week Ago Thread Starter
Charlie DuBois's Avatar
Joined: Nov 2019
Location: Orange
Posts: 10
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.


Charlie DuBois is offline  
Sponsored Links
post #2 of 4 (permalink) Old 1 Week Ago
GT Member
Charles Reeves's Avatar
Joined: Sep 2010
Location: Knoxville
Posts: 1,477
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.

Charles Reeves is online now  
post #3 of 4 (permalink) Old 6 Days Ago
73 351Q's Avatar
Joined: Nov 2018
Location: eastern iowa
Posts: 21
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
73 351Q is online now  
post #4 of 4 (permalink) Old 6 Hours Ago
GT Member
Classic Member
5.0L Member
Grimbrand's Avatar
Joined: Nov 2016
Location: Sedgwick
Posts: 2,206
Send a message via Skype™ to Grimbrand
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!

I smile a lot. It makes people wonder what I'm up to...
Grimbrand is online now  
Sponsored Links

Quick Reply

Register Now

In order to be able to post messages on the Ford Mustang Forum forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.


Confirm Password:
Email Address
Please enter a VALID email address for yourself, otherwise you will not receive the necessary confirmation email needed to confirm, validate and activate your new AFM member account.

Failure to provide a VALID email address, will result in the cancellation of your new AFM member account registration.

Email Address:


Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are On

For the best viewing experience please update your browser to Google Chrome

Search Engine Friendly URLs by vBSEO 3.6.1