John Larkin
05-14-2005, 10:56 AM
N/A Regals and most post-1990 vehicles use a quick-take-up master cylinder. Their reason for being developed was to be used with low-drag calipers (ones that retract farther than prior versions for increased fuel economy, i.e. low drag). When you do a vacuum brake swap, this is the master cylinder you are most likely using (aluminum body, plastic reservior, all on a sharp upward angle). The problem can be two-fold: brakes have a long travel in the pedal or continue to sink to the floor.
The first step to be performed is block the front and rear brake hoses with clamps. Then start the car and see if the pedal has firmed up. If yes, then remove front or rear clamp(s) one end at a time and you find the offending end of the car. Then inspect that end of the car to see if there is a physical deficiency in adjustment or operations. If none found, this is the following is a test to see if the quick-take-up valve is defective.
In the rear reservior chamber, there is a plastic mound with a hole. This is the quick-take-up valve port. You must seal this port to test the valve. Take a drywall screw, insert it into the port (it's plastic so be careful but it will seal; drywall screws are nice and sharp + thin enough to work, do not use a sheet metal screw), and tighten 1/4 turn. Then start the car and apply the brakes ONE TIME. You only get one application because the system must divert pressure through the port you plugged and it will multiply with repeated brake applications. If your pedal is nice and firm (with all brake hose clamps removed), you have a bad master cylinder i.e. bad quick-take-up valve. It is not servicable so replacement of the valve is necessary.
Ensure that quick-take-up master cylinder is bled properly before just changing it out. You must wait 15-30 seconds between strokes when bench bleeding so the master cylinder's large step bore can refill and get ALL the air out.
My comments are a synopsis of this article:
http://www.babcox.com/editorial/bf/bf60133.htm
Good luck. This is almost the end of a frustrating ordeal in sorting out my own brake pedal problems. I still have to rebleed and readjust my rear drums to get a little more pedal slack out.
The first step to be performed is block the front and rear brake hoses with clamps. Then start the car and see if the pedal has firmed up. If yes, then remove front or rear clamp(s) one end at a time and you find the offending end of the car. Then inspect that end of the car to see if there is a physical deficiency in adjustment or operations. If none found, this is the following is a test to see if the quick-take-up valve is defective.
In the rear reservior chamber, there is a plastic mound with a hole. This is the quick-take-up valve port. You must seal this port to test the valve. Take a drywall screw, insert it into the port (it's plastic so be careful but it will seal; drywall screws are nice and sharp + thin enough to work, do not use a sheet metal screw), and tighten 1/4 turn. Then start the car and apply the brakes ONE TIME. You only get one application because the system must divert pressure through the port you plugged and it will multiply with repeated brake applications. If your pedal is nice and firm (with all brake hose clamps removed), you have a bad master cylinder i.e. bad quick-take-up valve. It is not servicable so replacement of the valve is necessary.
Ensure that quick-take-up master cylinder is bled properly before just changing it out. You must wait 15-30 seconds between strokes when bench bleeding so the master cylinder's large step bore can refill and get ALL the air out.
My comments are a synopsis of this article:
http://www.babcox.com/editorial/bf/bf60133.htm
Good luck. This is almost the end of a frustrating ordeal in sorting out my own brake pedal problems. I still have to rebleed and readjust my rear drums to get a little more pedal slack out.