Can Bad Caliper Piston Cause Mush? (Not a noob, brake experts needed)

Okay, before I start, I'm not a noob, I've done plenty of work on bikes and brakes for that matter.

I've had pretty mushy/spongey brakes for about a year now but decent enough to ride the trails behind my house. Recently, I found myself with some money so I thought I'd do something about them so I bought an OEM Master Cylinder Piston and Seals, Brake Pads, and a Steel Braided Brake Line.

I installed all of this, bled the crap out of it, only to find out that brakes felt the exact same as before. (I know how to bleed brakes) I got curious and took the caliper off and realized that one piston was doing the majority of the work. The other would move but not much. I then pushed them all the way in and the "bad" one wouldn't move at all after pulling the brake lever.

I know this needs replacing but can this cause mushy brakes or is it something else that I can't think of? I thought usually a bad piston in the caliper made the brakes stick. I just can't logically think of why it would act spongey. (Btw they aren't leaking fluid and the rotor isn't bent).

Edited by jpkoepse

I've also read somewhere that for some reason the master cylinder can suck the piston back in so the piston has to cover alot more ground before hitting the rotor. What would make it do that? Just trying to think of reasons to explain this....

Edited by jpkoepse

no ideers???

Just some suggestions here. First, gravity feed the fluid through the master cylinder to the caliper. Make sure fluid does get there. Second. You may have an air pocket in the master cylinder. Tighten up the bleed fitting at the caliper. And pump the lever and loosen the line coming from the master cylinder. Make sure fluid is coming out of the line going to the master cylinder. It may be a banjo fitting, possibly a 5mm allen head. Spongy brakes are from air in the system. Make sure you use Dot 4 fluid. I'm sure you gone this route. Just maybe a different approach may help.

Yeah, I've tried this too :thumbsup: gravity bleed and cracking the banjo bolts.. still super squishy.

I know that my seals on the calipers need replacing but I figure if one of the pistons is binding up a little, the pressure will still equalize once the other piston is pressing on the rotor... so I don't see where that could lead to mush...

If the pistons aren't leaking they won't cause a spongy lever.

are the slider pins holdiong up the pad on one side? that will cause mush since now if on one side the pad is stuck on the pins, the other side has to go twice as far to take up the gap. if the one piston isnt moving as well it sounds like its time for a caliper or a rebuild. usually the pistins get stuck in the out position causing a drag, but they can get stuck in the in position and if they are stuck that bad they wont move also causing mush.

when you have the caliper off and pads out, and you see the one piston come out..if you hold that piston, will the other piston go out when you squeeze the lever? It is not unusual for one piston to move and the other to stay. Just check they both move by holding one then the other.

When you start threads like this, it really helps to have the year/make/model of the bike too .. Ya know? A new bike one could eliminate some parts, vs. a 20 yr old bike.. could be many things. Also helps to know if you used DOT 4 fluid as you should..

ALSO, how do they work when you RIDE? Many dirt bikes have a slightly spongy feel up on the stand, that is cause they don't take much pressure to lock up (on a dirt bike). My 08crf450 is an example. You can pull the lever to the grip (if you squeeze hard), but when riding it never comes that far before locking.. I adjust my lever to the point it almost comes to my kuckles when i pull hard (while riding) with 2 fingers.

It's actually a 2003 CR-125. It's not getting caught up on the pins from what I can tell(They looked flush with the pistons). I was using DOT 3 brake fluid. I didn't think there was that big of a diff between dot 3 and 4. I was actually using a buddies brake fluid (DOT 3) which he used and his brakes felt awesome.

I'm thinkin that the piston is cocked a little sideways when it's in the caliper. If I held the good piston in, the other wouldn't really move. So that can cause mush? I thought the pressure would either equalize or all the pressure would go to one piston and would still provide plenty of resistance against the caliper (it would just take less of a pull)

Yeah and I'm almost positive it's not leaking fluids

Edited by jpkoepse

@Melk Man

I replace the master cylinder piston, brake line, and pads. Not the caliper pistons

And when I ride, sometimes they feel better than others but i'd say 80% of the time they're crap

Edited by jpkoepse

the pins will sit flush but if there is a tear in the boot or a small hole it will let water, dust, mud, ect in and can cause rust on the pin causing it to stick. it sounds like a caliper or the pins thats your problem. honda also reccomends dot 4 fluid and you shouldnt mix dot 3 and 4 with each other.

always start with new fluid, brake fluid begins attractting moisture the second the cans seal is broken.

Make 100% sure that your brake disk is not bent, similar thing happened too me.

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