New master cylinder problems

loki_racer

loki_racer

Well-Known Member
Joined
Sep 7, 2005
Location
Pittsburgh, PA
I installed a new dual diaphragm brake booster and master cylinder in my 91 XJ. I followed the instructions about bench bleeding the cylinder before installing.

Once installed, I attempted to bleed the brakes but ran into a problem. With all the bleed nuts closed and the reservoir cap on tight, the brake pedal will depress all the way to the floor with extremely easy.

If I open up the bleed nuts and have someone pump the brakes almost no fluid comes out. 20 or 30 brake pedal pumps equals maybe a soda cap full of brake fluid.

Any pointers about what to do next would be greatly appreciated. Thanks.

Someone recommended using a pressure bleeder, but I've been unable to find an adapter that fits the reservoir and I'd rather not pay $80 for a tool I'll probably never use again.

Also, I had to flair and bend my own lines. I assume they are done correctly because fluid does come out when the bleed nut is open, it's just not very much fluid.
 
I bought a Auto Zone Master cylinder and it was already bad. Sounds like you might need to swap that one out and start over by bench bleeding it again. I used a brake bleeding syringe to bleed my master. They're pretty cheap and it works much better than doing it the conventional way. Did you start bleeding the brakes by pumping them 10-20 times then starting at the farthest point from the master cylinder? In your case, farthest point would be the passenger rear brake.

IF you let all of the fluid drain out of the lines, it will take a good while to get all the air through the lines before you'll get much fluid. When I did mine, I pumped like crazy for about an hour for about a soda can or less worth of fluid. Come to find out my "New-remanufactured" master cylinder was bad out of the box.
 
If the new MC isnt DOA as suggested above...how much air did you let get in the lines? do you know?

Id start with a gravity bleed topping the MC regularly until you have a good stream, then move on to pump and open bleeds. Its not uncommon for the pump and crack bleeder technique to take 10-15 times at the farthest point to start getting good fluid after a MC replacement at times.
 
The MC wasn't reman'd. I'm way past the point of being able to return it and getting another one could take months.

I try the things Ron mentioned next weekend and check back. Thanks for the ideas. I have no idea how much air got in the lines.
 
If the MC inst a bad one...as Ron said, gravity bleeding is the best way to start. Especially if the system was pretty empty!

Often as soon as you start to bleed, with very little pressure in the system, the combo valve will shift to one end and make bleeding very difficult to start. (Usually shifts to the front since you generally open the rear first.) When you start pumping pressure and open one end, the combo valve sees that as a brake failure and shifts. By gravity bleeding, you fill the system, and that allows the combination valve to equalize. The entire system will pressurize quicker and more evenly and they will start to bleed easier.
 
Just gonna toss this out for future reference...
How about the booster pushrod? Any chance it is adjusted too short? Compare the depth of it vs. the old one.
 
Jody Treadway said:
Just gonna toss this out for future reference...
How about the booster pushrod? Any chance it is adjusted too short? Compare the depth of it vs. the old one.
Click to expand...

X2
I recently had one of these on Mike's "Bass boat" project when we swapped to a different MC!
 
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