[tc33]

I went through the same problem on my 33 Plymouth. Weak pedal after replacing the MC.

I bled the turkey four times with no luck. I also forgot to bench bleed the new MC.

Finally I used a bleeding kit with a container to prevent any back pressure air to get in. Bled the system at all four wheels. It looked like there was no air bu the pedal firmed up and I've had no problems since.

Air will get in the system and its a b***h to get out. Hang in there and keep bleedin !!!

[troy-curt]

By pumping the pedal and getting a firm pedal and holding it, then it slowly going down is an indication of air still in the system. If it pumps up then the rod should be alright. Make sure the calipers are on right with the bleeders up on each side, if not they will never purge all of the air out.

Troy

[yesgo]

The M/C for ABS and nonABS are of the same design on the S10. Both are 24x36mm step bore made to match 63mm front "low drag" metric calipers and 3/4-7/8 rear cylinders. It is also designed to work with an S10 combination valve. The master contains no residual valve. The largest resivoir should go to the fronts.

Only the ABS control valve has the ability to bypass fluid.

The combination valves may differ, as they do between disc/drum and disc/disc, and different weighted vehicles.

A vertically mounted valve has a different bleeding procedure than a horizontal mount, as it can trap air.

A disc/drum valve can contain a valve for the fronts which holds off pressure until the drums catch up, and shouldn't be used on a disc/disc system.

The difference in the rear brake part of the valve is how much pressure it restricts and when. There also might be a residual valve in it....

Sorry I have no advice. Just info of which you might already know.

[yesgo]

I forgot something else about the step-bore master. I dont know if this will help with your diagnosis or not...

The check valve (QTU valve) inside the master closes at about 100psi. While the valve is open, the large (1 7/16") bore delivers high fluid volume with little pedal travel. After the valve closes, pressure is delivered by the small (15/16") front bore.

The step bore is deigned to operate low-drag calipers without giving a mushy pedal.

I think this negates the need for a front hold-off valve in these systems, as it would probably screw it up.

[RickB1B]

This is great information and may help me out. I noticed the other day I had the old style front calipers. I then bought new ones, but the parts store gave me one of the old ones and one of the new (metric) ones. When the pressure comes off the brakes I noticed the old one is dragging pretty good.

I will update this and see what happens.

[yesgo]

If you have the s10 master and combo valve, and no other valves, it shouldn't matter whether you have low-drag clappers or not, but they should match side to side.

I just mention that thing because the step bore can sometimes give a firm pedal and hide problems during diagnosis. If you have air trapped, the high volume part might be hiding it until the valve opens... I dont know.

You just cant go the other way and use a regular master with low-drags, unless you add a residual valve to hold the caliper pistons out. The low-drags retract the pistons further and need the extra volume.

Hope I'm not talking in circles. Too much info can be worse than too little sometimes.

The problem of having a good pedal that sinks after 20 seconds suggests a fluid bypass somewhere, or a restriction with air after it, something like that. Something opening up after 20 sec. and letting fluid by and/or air compress. Strange problem. Almost has to be trapped air.... I dont know.

[RickB1B]

Guy's thanks for all the help. I finally got all the air out and took the car for a drive today. Brakes work fine. I'm pretty sure it just needed a lot more bleeding. You all definitely helped me in the trouble shooting, by helping me eliminate other possible causes.

Rick