Do these have breather holes in the caps? I had a scary moment today - started the car and rolled doen the drive towards the road, dabbed the brakes ..... and my foot went to the floor!!

Several frantic pumps later I regained enough braking to come to a gentle halt inches from the curb [phew] and then gingerly drove the car back up the drive to investigate (and get a change of pants)

I looked at the brake fluid level - all was ok and I couldn't see any leaks so I tried the pedal again and after a couple of pumps it returned to normal.

I noticed that my alloy reservoir cap does not have a breather hole so I reckon it must have built up a vacuum, preventing fluid filling the m/c properly. Removing the cap removed the vacuum and the problem .......... what do you think?

How can I put a breather into the cap without letting moisture into the reservoir?


[Edited on 4-7-07 by RazMan]

Eeeek.

Rally design do a rubber diaphram which I believe allows a breather hole without air coming into contact with the fluid.

http://www.rallydesign.co.uk/catalog/product_info.php?cPath=562_1357&products_id=4058

How much moisture is going to get in through the smallest hole you can make to do the job? I'd make the hole and keep an eye on the situation to see if it happens again.

My concern is that brake fluid is hydroscopic and will absorb moisture from air really easily, so I am reluctant to simply drill a 1mm hole in the cap.

The RD diaphragm won't fit my res (I didn't think of this when I designed the reservoir) Maybe one of those motorbike tank breathers would work - they need a small negative pressure to open so reducing any exposure to outside air.

Drill hole in cap. Fit small pipe fitting into hole. Attach small bore pipe to pipe fitting and then attach pipe to a container filled with silica gel/granules to absorb any moisture. JD & no worries about moisture.

Aha - lateral thinking

Floor mounted pedals?

I think it is a case of the fluid going back into the cylinder due to gravity.

A residual pressure device made by willwood (rally design stock them) will maintain 2psi, stopping fluid from draining back.

Darren

Thats an interesting point Darren. My m/c might be slightly lower than my calipers but the distance can only be a few mm.
Worth looking into though It might even reduce the amount that the caliper pistons retract, reducing the amount of slack in the pedal.




[Edited on 4-7-07 by RazMan]

Raz never heard of this or expereinced it myself and ultima ones are floor mounted and use cheapie Gerling ones....

I think its down to the expansion in the resovouir.