Manual to power brakes problem
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 40
Likes: 0
From: Brownstown Michigan
Manual to power brakes problem
Hello. I was doing my own convertion from manual to power brakes with the help of a friend when things have gotten from good to bad. I own a 1970 Stingray convertible 350/350 4 speed. I started bleeding the system from the master cylinder down to the proportioning valve and all of the calipers. At first, I was getting low pressure from the LR and RR calipers, then with the help of vacuum bleeder I was able have good fluid pressure in all of my calipers. The problem I'm having is that the rod that came with the booster MC combo was a little longer that it should. When I mounted the master cylinder to the booster the push rod is hitting the MC and was pushing the MC about 1/8 to 1/4 of an inch pressure. With the pedal adjustment is adjusted to maximum away from the master cylinder. As a result, and after 20 min of driving, all of my calipers locked up on me. I got the car back to my garage and called the tech support and he suggested to shave 1/8 to 1/4 inch off the MC push rod, which I did and the locking up problem is solved. I rebled the system with the car off and got good pedal. However, when I turn the car on the pedal goes down to the floor and vehicle barely stops. My friend suggest to replace the master cylinder. Is he right? Please help. Thanks. Tony
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
You may have two problems. First you may have the wrong master. What year car is it from or for and was it bench bled before installed?
The other thing, is on your pedal where the rod connects. Do you have the pedal rod pinned to the same hole as the manual master or did you use the lower hole. Manual brake cars have an upper hole on the pedal and a lower hole. The upper was used with the manual master for great leverage. Power brake masters pin to the lower hole.
Check that and see where you stand.
Also, I have a feeling the master does either need to be replaced or rebuilt or there is just too much air in the system and you can't get good pressure at the wheels.
This may be due to the fact that the master is mostly depressed and not getting full action when you pump the brakes.
The other thing, is on your pedal where the rod connects. Do you have the pedal rod pinned to the same hole as the manual master or did you use the lower hole. Manual brake cars have an upper hole on the pedal and a lower hole. The upper was used with the manual master for great leverage. Power brake masters pin to the lower hole.
Check that and see where you stand.
Also, I have a feeling the master does either need to be replaced or rebuilt or there is just too much air in the system and you can't get good pressure at the wheels.
This may be due to the fact that the master is mostly depressed and not getting full action when you pump the brakes.
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 40
Likes: 0
From: Brownstown Michigan
manual to power brakes problem
Thank you for replying. I have a 1970 stingray convertible. I did bench bled the master cylinder before installing. There is only one hole for the pedal rod on the brake pedal. In addition, I crimped all of my brake hoses near the calipers with vice grips and started the vehicle. As a result the pedal was rock hard indicating no leaks on the master cylinder. So I suspect the air is somewhere in the calipers. We bled the system again and again last night and got solid fluid coming out of the calipers except for the left front. Should I replace just the left front first to see if that is really the problem before buying all four calipers? Thanks.
You may have two problems. First you may have the wrong master. What year car is it from or for and was it bench bled before installed?
The other thing, is on your pedal where the rod connects. Do you have the pedal rod pinned to the same hole as the manual master or did you use the lower hole. Manual brake cars have an upper hole on the pedal and a lower hole. The upper was used with the manual master for great leverage. Power brake masters pin to the lower hole.
Check that and see where you stand.
Also, I have a feeling the master does either need to be replaced or rebuilt or there is just too much air in the system and you can't get good pressure at the wheels.
This may be due to the fact that the master is mostly depressed and not getting full action when you pump the brakes.
The other thing, is on your pedal where the rod connects. Do you have the pedal rod pinned to the same hole as the manual master or did you use the lower hole. Manual brake cars have an upper hole on the pedal and a lower hole. The upper was used with the manual master for great leverage. Power brake masters pin to the lower hole.
Check that and see where you stand.
Also, I have a feeling the master does either need to be replaced or rebuilt or there is just too much air in the system and you can't get good pressure at the wheels.
This may be due to the fact that the master is mostly depressed and not getting full action when you pump the brakes.
Thread Starter
Intermediate
Joined: Jul 2007
Posts: 40
Likes: 0
From: Brownstown Michigan
manual to power barkes problem
Hello. I shaved off approx. 1/8 of an inch off the push rod and that seemed to fix the brakes locking up on me. But still the pedal still goes down to the floor regardless of countless amount of bleeding the calipers. Still looking for answers. Thanks for your help. Tony
Burning Brakes
Joined: Nov 2005
Posts: 1,143
Likes: 0
From: Leander Texas
It sounds as if you still have air in the system. If you clamp all the brake hoses with the proper clamps (vice grips can crush the hose internally) and have a hard pedal the problem is in the calipers. Release one clamp at a time starting at the furthest caliper from the master, and work toward it testing pedal feel as you release each clamp. If you get a spongy pedal when the clamp is removed, this indicates that air is trapped in the Caliper, or that the caliper has a leak.
Do you have any fluid lose?
Do you have any fluid lose?
Burning Brakes
Joined: Feb 2006
Posts: 1,173
Likes: 5
From: Paducah Ky
Sounds like you still have air in the system if the pedal goes away after the car sits. Air is a real problem to bleed from these systems so be patient. I like to take my calipers and elevate them as much as possible with the bleeders pointed up and allow to sit overnight. The air will try to rise to the highest points. Crack the bleeders oneat the time and allow gravity to push the air from each. Another possibility is that you have overheated and damaged the piston seals with the improper Master setup that didn't allow the pressure bleed port to open. Once you have this corrected you will need to address the pushrod mount location to the brake pedal. The manual mount location is too high for you to get the proper travel to the brake booster. The lower hole may be occupied by the brakelight switch. If not, you will need to drill the proper hole.
Safety Car
Joined: May 2005
Posts: 4,176
Likes: 7
From: Jennings LA
no there is two the other one has the brake light switch actuator bracket in it now. you have to swap locations. and you will need a new bracket. like durango said this could be a problem. the actuator rod for the booster is lower on the pedal than the original non power master cyl rod was 
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