About every four weeks I get my bike out the car to find some air has managed to sneak into my rear brake.

First pull of the lever goes to the bar then subsequent pumping temporary restores normal function. Stop pumping and lever goes to the bar again. So this is air in the system, right?

Bleeding fixes this but I have notice the fluid looks pretty manky in the back brake, when compared to the front, after only being in there a few weeks.

Any ideas what is causing this???

It’s always the rear brake, it works flawlessly the rest of the time and this only happens after a car journey.

It’s ruining too many rides! Driving to wales to find your rear brake is playing up is most annoying!

Sounds to me like you have a leaking seal… Is the fluid black in colour by any chance?
The master cylinder has a diaphragm under the top cover, try changing that one first.

I feared it might be a seal. They’re 3 year old hope tech M4’s. The bike gets ridden 4 times a week every week and they’ve not caused any problems so far.

The fluid was indeed black. I changed the lever diaphragms recently, but I’ve got a couple in my spares box so I’ll try popping some fresh ones in.

When I put the bike in the car I lay it on its side so the rear brake lever is in the footwell. I thought this could somehow be causing my problem.

The black is the dye coming out of the seal material, Hope recommend replacing the seals in the whole brake system every 2 years, which is actually pretty easy with the tools and half a day to do it and the seals don’t cost a lot… The alternative is Hope will do the job for you and chuck in a set of new pads for about £40. Give ’em a call see what they advise mate.

Wow really? Change seals every two years? Hah 🙂

2 sets of Hope brakes that are now… 8 years old. Bled them recently, the outgoing fluid was the same as the new stuff.

Will change seals next time.

Sounds like a top diagnosis Cardo. I’ll look into the cost of the tools/parts required and make a decision. Thanks!

Change the seals every 2 years

Wow. I’ve got some original M4’s on the HT that are at least 6 yrs old (probably more) & some X2’s on the FS which are about 4 yr old.
Neither have been bled since fitting & both work perfectly every time!

The only time I’ve had a problem with Hopes was when I stored/transported the bike upside down. The small amount of air in the reservoir was heading north into the system. I think that was with some original Mini’s.

Tools for seal change are pretty cheap. Cocktail sticks are best tool.

Is your caliper above the master cylinder when in the car?

Could just be the air from the cylinder, which is fine, migrating to the caliper.

Bet. The laying the bike Down is also a factor… There is some Air in the Mc. Launig it Down on its left side Entouragen it to make Its way into the Hose…

Dot 5.1 synthetic Works well… Kind on the seals.

Dot 4 is fine.