Front brakes losing pressure rapidly - SV650.org

mackemforever · 23-08-12, 01:05 PM

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 · 23-08-12, 01:19 PM

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 · 23-08-12, 01:38 PM

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 · 23-08-12, 03:18 PM

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 · 23-08-12, 03:19 PM

Red rubber grease, as YC suggested

yorkie_chris · 23-08-12, 03:35 PM

Quote:

Originally Posted by mackemforever

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 · 23-08-12, 03:48 PM

Does the deteriation happen when the Bike is stationary?

mackemforever · 23-08-12, 03:49 PM

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

NTECUK · 23-08-12, 03:56 PM

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

23-08-12, 04:09 PM

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

23-08-12, 01:05 PM	#1
mackemforever Member Join Date: Aug 2010 Posts: 144	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.

23-08-12, 01:19 PM	#2
maviczap Member Mega Poster Join Date: Mar 2007 Posts: 9,096	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. __________________ We're riding out tonight to case the promised land Make everyday count RIP Reeder - Jolly Green Giant and comedy genius

23-08-12, 01:38 PM	#3
yorkie_chris Noisy Git Mega Poster Join Date: Apr 2007 Location: Halifax/Leeds Posts: 26,645	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. __________________ Currently Ex Biker Now rebuilding a 63' fishing trawler as a dive boat

23-08-12, 03:18 PM	#4
mackemforever Member Join Date: Aug 2010 Posts: 144	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?

23-08-12, 03:19 PM	#5
maviczap Member Mega Poster Join Date: Mar 2007 Posts: 9,096	Re: Front brakes losing pressure rapidly Red rubber grease, as YC suggested __________________ We're riding out tonight to case the promised land Make everyday count RIP Reeder - Jolly Green Giant and comedy genius

23-08-12, 03:48 PM	#7
NTECUK Member Mega Poster Join Date: Apr 2011 Location: essex Posts: 5,314	Re: Front brakes losing pressure rapidly Does the deteriation happen when the Bike is stationary? __________________ Raisin cookies that look like chocolate chip cookies are the reason I have trust issues.

23-08-12, 03:49 PM	#8
mackemforever Member Join Date: Aug 2010 Posts: 144	Re: Front brakes losing pressure rapidly Ntecuk, I've already answered that, yes it does.

23-08-12, 03:56 PM	#9
NTECUK Member Mega Poster Join Date: Apr 2011 Location: essex Posts: 5,314	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 __________________ Raisin cookies that look like chocolate chip cookies are the reason I have trust issues.

23-08-12, 04:09 PM	#10
speedyandypandy Guest Posts: n/a	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.

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