Tractor Repair: 585 case international brakes, clear plastic tubing, bolt holes

Question
i can not keep air from entering the braking system on the tractor. i have rebuilt both master cylinders and put all of the o rings in the brake pistons. do you have any idea why i can not keep the brakes air free. thanks

Answer
Hello,

  There are two scenarios that can take place.  1) You keep bleeding the brakes and always have more air bubbles.  You might get the brakes to work for a short while, but then lose them again after it sits overnight and you have to bleed them again getting still more bubbles.  2) You keep bleeding and can't seem to get any more visible bubbles, but you still can't get a decent pedal on the first stroke and you have to pump the pedal to get any brakes.  In the first situation, there really is air present in the system most of the time and you can't get rid of it for good.  You have done all the right things so far.  It normally is the o-rings on the brake pistons that leak oil from the brake cylinder into the rear end of the tractor.  You didn't say how you installed the o-rings, but unless you have a special tool to press the pistons back in PERFECTLY straight, you will damage the new o-rings every time.  A simple tool can be made from a couple pieces of steel welded into the shape of a T.  Drill holes in the ends that line up with some brake housing bolt holes.  Oil up the o-rings and using 3 bolts, slowly and evenly turn each bolt a little at a time keeping the piston absolutely square with the bore.  Keep turning the bolts until the piston is drawn in straight and even.  If one part of the piston gets slightly ahead of the rest, the o-rings will definitely be scraped, nicked, or cut.  It is impossible to get the piston in perfectly straight without using this method.  When bleeding the brakes after everything is assembled and the tractor filled with oil, attach some clear plastic tubing onto the bleeder screws and rout them into the oil fill hole on the back of the tractor.  Start the tractor and slowly push each brake pedal one at a time, then both together, then hold one pedal down, and push the other a few times.  Keep doing this until no more bubbles show in the tubing.  It might take a while so be patient.  Don't go too fast or the master cylinder bottle won't stay full enough.  Close the bleeders and check the pedal operation.
In the second scenario, there really is no more air in the system, but it acts like there is because the pedal is soft and goes way down until you pump it a couple of times.  In this case, the brake disks or backing plates are slightly warped or cupped, causing the piston to be pushed back in too far after the brakes are released.  It doesn't take much, sometimes the warpage is so slight you cant't even tell until you check with a straight edge.  To check where a problem is for sure, you can remove the brake lines from the master cylinder and cap the fittings on the master cylinder.  If the problem disappears and the pedals stay hard even overnight, the problem is in the brakes.  If the problem is still there, it's in the master cylinder.