Rats. So much for the easy solution.
Is it possible the pistons just have not been pumped out far enough yet? You said the brake pedal is good when the front hoses are pinched. I have never done that so I do not have a feel for how that compares to normal operation. The pedal should be higher and harder than normal. There has to be some fluid movement, but it sounds like you are hitting the two extremes, from too much fluid movement, (low pedal) to no fluid movement, (pinched hoses).
Normally it is a very bad idea to push the brake pedal all the way to the floor because crud and corrosion build up in the lower halves of the bores where the pistons do not normally travel, and that can rip the rubber lip seals, but that warning applies to master cylinders more than about a year old. That should not be a concern with your new one, but if you were following that precaution, push the pedal a little further to run the pistons out to adjust them. Watch those pistons too while a helper works the brake pedal. The pistons ride on and are sealed by a rubber square-cut seal. Besides sealing, it is designed to stick to the piston, then bend, or flex, when the piston starts to move out. When you release the pedal, that seal wants to straighten out, and doing so pulls the piston back into the caliper just enough to release the pads from rubbing on the rotor. Watch if the piston is retracting too much. You should be easily able to see it come out a little when the brake pedal is pumped, but once adjusted all the way out, you will have to look really closely to see it retracts a very tiny amount.
If these calipers sat on the shelf for a long time after being rebuilt, it is possible the brake assembly lube dried out and the square-cut seal is sticking to the piston too aggressively. That will cause it to flex too much when you push the brake pedal, then the piston will get pulled back in too much when you release the pedal. Retracting too much means it has too far to move out next time, so the pedal is too low again. The simple solution for that is to just push the brake pedal further. Once the piston moves out a good eighth inch or so, the seal will have no choice but to let go and let the piston slide through it. Under normal conditions, that is how all disc brake calipers self-adjust over time.
Also, watch if the entire caliper moves or shifts position when the brake pedal is pumped. That would point to worn guide pins, worn rubber isolators, or some other mounting problem. As I mentioned earlier about loose wheel bearings, mounting problems only cause a low pedal after driving and bumping down the road. But while a wobbling rotor will push the piston in while driving, caliper mounting problems can allow the caliper to turn, which allows the piston to come out too far, thus the low pedal, then there can be enough return force to push the caliper back straight when the pedal is released, and that pushes the piston back in too far, and you have the low pedal again next time. That can occur in the shop when standing still. You will see that movement when you watch the caliper as your helper pushes the pedal.
For my last comment of great value, have you actually driven the truck yet, and if not, are you doing this work with it up on jack stands with the rear axle hanging down? I know we are thinking this problem is related to the front brakes, but there is one more thing to look at. Trucks and minivans can have a really wide range of loading in the rear, from empty to heavily-loaded. All cars have a proportioning valve in the combination valve below the master cylinder that limits the amount of brake fluid pressure to a point that prevents easy rear-wheel lock-up under hard braking. That valve is very carefully selected for all the optional equipment and weight distribution for each model. It is impossible to tailor that one-size-fits-all-conditions valve to vehicles that will be loaded differently at different times. On those vehicles, you will find a height-sensing proportioning valve on the rear frame, with a link attached to the axle housing. When these vehicles are raised on a hoist or in such a way as to let the rear axle hang down, it looks to that valve as though the vehicle is lightly loaded. It is going to restrict fluid flow to the rear wheels, and that can reduce flow so much that bleeding goes so slowly that air can become trapped in a high spot and not be flushed out by the low force of the fluid flow. If you see brake fluid dripping very slowly from the rear bleeder screws, jack the rear axle up, then try again. Also, have the cap loose on the reservoir so fluid flow does not create a vacuum in there that restricts that fluid from flowing freely.
It takes quite a bit of air in the line going to the rear brakes before you will notice a low brake pedal. You might have nothing actually wrong in the front, and pinching the hoses eliminates all the normal volume of fluid flow to them, and that is offset by air in the rear line.
Wednesday, April 14th, 2021 AT 10:15 AM
(Merged)
