No pedal pressure after front disc swap?

Mike , ummm,a butt load of air is never good.

 

It was blowing bubbles like a baby in a bath tub.

 

Sucking air in somewhere. I'd suspect the m/c but double check the fittings at the calipers.


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Yep m/c it was. I upgraded to the matching disc model to go with the calipers. It stops well, but I am not sure how it's supposed to feel. The drums were very positive and had a solid feel. The discs seem to have more squish, not pedal to the floor and still need more brakes squish, but softer than I imagined. Maybe still some air in the system? How are manual discs supposed to feel?

 

Pedal should be firm. Sounds like air still in the system. Make sure the bleeders are at the highest point on the calipers or you won't be able get the air out.


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have you tried a vacuum bleeder? soft pedal =air

 

Thanks thats what I was thinking. I'll keep at it, at least i've made some progress.

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The fun never ends with this project! I have worked out the other issues I was having, lack of pedal pressure, air in the lines, etc. I replaced the M/C with a disc model and verified that the front brakes are attached to the front port. NOW the front brakes have excessive drag after a few (less than 5) miles of driving.

Could this be still an air issue, defective master cylinder, the stock distribution block, or ......? The calipers are remanfactured units and the rubber lines have been replaced with new steel braided lines, the hard lines have not been changed.

Any thoughts? bunny with a pancake

 

What's the application of the master? Not all masters have the reservoirs for the front/rear in the same place. Some are rear brake, front reservoir, some are rear brake, rear reservoir. There's a chance your new master has switched the reservoirs front-to-rear. Dragging discs sounds like there's still a residual pressure check valve behind the brass fitting in the master. You may find you need to swap the lines on the master front-to-rear.

 

FYI - if it's a large/small reservoir master the larger reservoir is for the disc brakes, the small one for the drums.

 

I am using a small/large reservoir disc M/C that, according to FLAPS, is the mate to the calipers I am using. (late model year '78 jeep cj stuff) I confirmed that the lines are attached to the correct reservoir, when I bled the calipers the larger reservoir was drawn down. My first thought was the residual valve, but since this is a disc M/C, shouldn't it have a 2lb valve and not a 10lb? I guess the easy check would be to take out the residual valve and see if that solves the problem.

 

Ok, so I am back at it. Went through the entire system again, double checked everything: claipers are installed correctly, the master cylinder is connected to the correct ports, bled the system to where the pedal is nice a solid no mush, no residual valve, and yet there is still too much pressure.

I thought I had figured it out, the keeper on the push rod was way thicker than necessary, so I corrected that problem allowing the push rod return to the proper position.

Put it all back together and took it for a test drive, in less than ten minutes the front brakes were so hot they locked up. So I am guessing that there must be a small amount of residual pressure build up causing the brakes to drag enough to create heat and then the problem compounds to the point of lock up.

To say the least I am stumped. Any thoughts are appreciated.

 

Make sure no lines, hoses, etc are near a heat source like exhaust. Make sure there is room for the fluid to expand. An overfilled master cylinder can cause what you're describing because pressure can build up as the fluid expands. Is there play between the pedal Pushrod and master cylinder? If not that is an issue.


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All the lines are in the stock locations away from the exhaust and any heat sources, I have filled the master cylinder about 3/4 full. The same as I have always done, the difference being a drum master vs a disc master. There isn't really any free pedal to speak of. I am using a modified drum master push rod, maybe I need to round up a push rod for a disc master? How about the wrong master cylinder?

I am using this one however the ports are on the driver side rather than the passenger as shown. http://www.quadratec.com/products/56213_04.htm I do have the two bold caliper set up.

 

I would investigate the lack of pedal free play before I went any further.


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Thanks Nick.

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X2 on what Nick said. I had that same issue with my front discs. I didn't have enough slack between the MC piston and the rod from the pedal. It only takes a little. If the piston can't return all the way it doesn't release the pressure in the MC.

 

x3 gotta have a little slack in the push rod or fluid backs up

 

On the hunt for the correct push rod for this master. The one I am using is from an eariler model and uses different type of retainer than required. Causing the piston to be pushed in slightly.

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You can buy or make an adjustable pushrods. If you google it a few hits come up.