Topic: Should the brake piston retract when no pressure is applied to the lever?

[jason teamshralp]

I just replaced my brake pads, and hose and new fluid but they still feel kind of spungy and the pads are locked on the disc. Not to where it won't spin but there is friction.

Is this normal?

I also have a leak now dripping down the lever. Time to replace/rebuild?

All of this from a 78 cb550k.

[laser145]

Did you get all the air out of the system? The brakes won't function properly until you do.

If the pads won't retract it's most likely a build up of scale behind the O-ring in your piston housing.  You would need to remove the pads, piston, and O-ring, then clean the scale out gently with a pick...

Were the brakes working before you changed the pads?

[BeSeeingYou]

.Make sure the return hole in the MC is clear.  You should be able to see fluid flow back in the reservoir.  It's the hole closest to the outlet port.  It looks as big as the other hole but is in fact very tiny. I use a strand of wire from a stainless steel wire brush to clear this hole.  If the MC is leaking it is time for a rebuild and do the caliper too.  The rubber seal is what retracts the piston in the caliper and as they age they lose their elasticity and junk builds up in the groove that it sits in.  The groove must be cleaned out too.  

[jason teamshralp]

I just got done bleeding the system until I used up all the fluid I had, still some tiny bobbles. I'm not sure if the piston was retracting before.

Then I cleaned out the caliper the piston had some pitting, but the caliper was pretty clean and o ring was good.

[laser145]

The piston will still function with a bit of pitting... but the groove that O-ring sits in needs to be clean.  It can be a pain, I've had to do the entire process multiple times until I've gotten it right.

When you get that groove under the O-ring clean, and put the O-ring back in, grease the piston with brake fluid and try to slide it back and forth in the cylinder...when it will slide, you should be good.

You'll need more brake fluid too...once the cylinder is retracting, the brakes will be soft until the system is properly bled.

[jason teamshralp]

Thanks I'll give it another go.

Would the leak in the master cylinder cause air to get in the system? both holes are clear inside the MC.

[gane]

Jason, Yes, & No.. the "seals"  in your' calipers to piston, are square in cross-section... and when working properly deform when piston travels, and when pressure is released draws piston back into caliper. since pad is free floating, it may "relax" but not nessicarily be be withdrawn.. typically pad will remain in contact w/rotor. but should'nt have appreciable drag. the most common, but by no means all, causes of dragging disc brakes are.  Build-up at piston to caliper,  Build-up of old brake fluid within seal land, sticking pads within caliper, caliper frozen to fork stancion (where applicible), and clogged relief  passage at master cyl. hope something here helps. G Ps, leaking at lever indicates leaking at M/C piston seal, & yes time for a M/C rebuild..

[jason teamshralp]

ugh! just remembered the guy at the store gave me dot 4 and told me it would be fine in my dot 3 system. could the mixture of the two fluids cause issues? the only minor amount of fluid would have been in the MC, everything else was new or bone dry.

[Hannibal Smith]

Jason, Yes, & No.. the "seals"  in your' calipers to piston, are square in cross-section... and when working properly deform when piston travels, and when pressure is released draws piston back into caliper. since pad is free floating, it may "relax" but not nessicarily be be withdrawn.. typically pad will remain in contact w/rotor. but should'nt have appreciable drag. the most common, but by no means all, causes of dragging disc brakes are.  Build-up at piston to caliper,  Build-up of old brake fluid within seal land, sticking pads within caliper, caliper frozen to fork stancion (where applicible), and clogged relief  passage at master cyl. hope something here helps. G Ps, leaking at lever indicates leaking at M/C piston seal, & yes time for a M/C rebuild..

This is spot on- When I used to do this kind of stuff (way back when) we even converted calipers to O-ring for faster/positive retraction due to rider demands, but in reality, runout will "push" the piston rearward enough that loss willl be almost non-existant............in a nutshell, if your piston seal is sound you are good to go. Your MC might need attention.

[BeSeeingYou]

DOT 3 and 4 are compatible.

[HondanutRider]

DOT 3 and 4 are compatible.

They are are supposed to be compatible; but, Honda recommended (in their Common Service Manual) that all the old DOT3 fluid be flushed and replaced with DOT4.

Here's the extract from that manual for reference: