Front brakes losing pressure rapidly

[mackemforever]

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.

[maviczap]

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.

[yorkie_chris]

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.

[mackemforever]

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?

[maviczap]

Red rubber grease, as YC suggested

[yorkie_chris]

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?

Red rubber grease is best.

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

[NTECUK]

Does the deteriation happen when the Bike is stationary?

[mackemforever]

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

[NTECUK]

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

[speedyandypandy]

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

[Jayneflakes]

Not sure if this will work for you, but get the brakes good and firm, then use a bungy rope to strap the brake lever on and leave it for a while. If you put a clean rag or piece of paper under the calipers, you will see if you have any fluid leaks. If the pressure drops in the system, the bungy will still keep pressure on the lever. This also encourages any air bubbles in the system to float to the higher point, usually the master cylinder reservoir.

When I did my brake lines a while back, we discovered that one banjo was leaking very slightly, so changed the olive and nipped them back up again. Problem sorted.

[yorkie_chris]

This also encourages any air bubbles in the system to float to the higher point, usually the master cylinder reservoir.


Think about that...

[mackemforever]

Thanks for the suggestions. A member on here that is local has let me have some of the red rubber grease he had and I'm going to strip down my callipers and rebuild them properly this time. I thought I could get away without using any proper grease, just using clean brake fluid, last time but it sounds like that might be the problem.

[yorkie_chris]

I think brake fluid is worse than nothing. It turns to crystalline crap and makes things worse.

[mackemforever]

Well I stripped the callipers down again, rebuilt them properly this time with a bit of red rubber grease on the seals and it worked a treat.

Made me realise that the brakes haven't worked properly since I bought the bike 18 months ago, when I stopped at the bottom of my road I felt like I was going to go over the damn handlebars!

Thanks.