Spongy Brakes

My brakes on the 'Cruiser, Double Puck Cleveland Disc, are bleeding down and losing pressure after a few days or so but I can't find any sign of the leak. No fluid on the hangar floor, none in the cockpit around the Master Cylinders, none around the brake lines anywhere. What gives? Leaking diaphrams inside the Master cylinders ? They're close to needing new pads again, would that have anything to do with it? Is there a rebuild kit for the Master cylinders?

Thanks

If your brakes feel spongy take a look at your brake lines. Chances are your brake lines are expanding with the pressure and will need to be replaced. Have someone sit in the cockpit and press down on the pedals while you feel the lines down along the gear legs if they expand that is where the spongy feeling is coming from.

It is a closed system and the fluid level will go up and down with temp and altitude. Should be several threads on this. That is why many have switched to the vented brakes with reservoirs like Steve's A/C and dakota Cub sell.

Spongy...

Spongy brakes can be an indication of air in the system. Bleed your system well, from the wheel to the master cylinder, see if that helps.

Mike

Flying a Seminole for training. After brakes are bled they feel firm and match each other. After a few flights left brake needs to be pumped by foot a few times to work correctly. Few more flights. Loses left brake altogether. No fluid leaks. Replaced linings same result. Discs are on order(they are just above minimum).

Any other ideas?

Check your master cylinders for sucking in air. ???

I changed the round rubber diaphrams recently in mine. They're located in the master cylinder. What a BIG difference. I noticed the packet for the new ones recommended changing them every two houndred hours.

There are a few newish pilots that fly my Cub so I'm hesitant to put on the newer fancier and better braking systems for fear of the aeroplane ending up on it's nose.

I would go for the newer systems. The discs have no servoing effect, unlike the originals, so have no tendency to grab. The vented masters with reservoirs have pedals always at the same spot, greatly enhancing safety. And while folks say the piston-style masters put out way more pressure, I have tried them one side at a time, along with disc brakes one side at a time, and have had very little difference in feel.

What you do not want for students is one brake to fail at the wrong time. You minimize that with reservoir masters. I am using Grove under a field approval, but have used Steve's as well, and they are good. You absolutely need a bleeder screw at the top front of the master.

Your pucks/pads should be touching the discs when coasting, when you release the brake pedal the vacuum is causing the pucks to move back into the caliper too much. If there's no fluid visable then you do not have a leak. Basically, you need to fully bleed the system again.