Front brakes losing pressure rapidly
 

For a change my bike has decided to throw a wobbly and give me something to do, so once again I'm back on here asking for a bit of knowledge!

For about the last 3 weeks my front brakes seem to have been losing pressure in the system over time. If I pump the brakes they are solid as a rock, but after only about 5 minutes there is noticeably more travel on the brake lever and they feel a bit spongy, and by the 15 minute mark I can pretty much pull the level straight in to the bars before the brakes start doing anything.

A couple of weeks ago I had my callipers off, stripped them down, cleaned them, and replaced all of the seals, not really expecting it to do anything, and it didn't.

So I'm guessing that it could only be either the master cylinder or the brake lines?

Is there an easy way of checking which it is? If not then which would be the best option to replace first?

Thanks.

Re: Front brakes losing pressure rapidly
 

Make sure you've bled all the air out of the system before going down the lines of thinking the worst

Any fluid leaks anywhere? No, then that rules out the lines.

Seals in the mastercylinder could have gone, but my thoughts are to still having some air bubbles in the system.

Re: Front brakes losing pressure rapidly
 

Doesn't sound like air. They'd be spongy all time.

What makes you suspect other 2?


Do they pump up, and stay pumped up when the bike isn't moving? If yes check discs for runout.
If no, repeat test with master cyl cap off.

I suspect brake seal drag is pulling pistons back in a bit. Try rebuild them again with some rubber grease on pistons.

Re: Front brakes losing pressure rapidly
 

As yorkie said, air in the system wouldn't cause them to get progressively worse over time, and they are not spongy at all to begin with.

The only reason why I suspected the other two components is because given that there are pretty much only those 3 things in the brake system it had to be one of them!

No they don't stay solid when the bike is stationary, absolutely no difference in how they are behaving whether the bike is stationary or running.

I'll follow your suggestion chris and strip the callipers down again and rebuild them. Will any grease do, provided it won't damage the rubber seals?

Re: Front brakes losing pressure rapidly
 

Red rubber grease, as YC suggested

Re: Front brakes losing pressure rapidly
 

Red rubber grease is best.

Silicon grease also, but I thought it was too slippy.

Re: Front brakes losing pressure rapidly
 

Does the deteriation happen when the Bike is stationary?

Re: Front brakes losing pressure rapidly
 

Ntecuk, I've already answered that, yes it does.

Re: Front brakes losing pressure rapidly
 

sorry :(.
When we have cars do that its useualy the pads sticking.
the high force bends the caliper and pads in ,Then they revover and go spongy

Re: Front brakes losing pressure rapidly
 

MC is drawing in air or have seal that's started to go, would be my 50 pence bet.