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Front brake master cylinder problem?
http://www.kawi2strokes.com/forum/viewtopic.php?f=21&t=13367		 Page 1 of 1
Author:  67lotus49 [ Sat Apr 14, 2018 9:58 pm ]
Post subject:  Front brake master cylinder problem?
So I'm trying to find the cause of a brake problem on my Aprilia RS250. Symptom is the front calipers will apply more and more pressure on there own
and try to lock the front wheel. I've had this problem before on other bikes when the caliper pistons stick and don't release freely. The first time this
happened I took apart the calipers and made sure everything was clean and not binding on release. After bleeding all is well for a short time and
then the free play in the lever goes away and the calipers start grabbing more and more. Could this be a problem with the master cylinder? The pistons
look perfect and show no signs of wear or corrosion, seals are pliable and everything is clean. This is a radial master cylinder type and the bike is
super low mileage (less than 400 miles) so it has sat for most of it's life in a guys living room. Could rubber brake lines cause something like this?
They feel pliable and not dry and brittle.
Author:  DGA [ Sat Apr 14, 2018 11:21 pm ]
Post subject:  Re: Front brake master cylinder problem?
The tiny master cylinder bleed hole is blocked, usually from being dirty, and you have some adjustment keeping the master cylinder piston pressed so the seal is covering the hole, but probably it's stuck so it won't return fully. Likely it needs the cylinder rebuilding, you can get lucky and 'carefully' poking a tiny piece of wire through the hole from the open cylinder cap. Don't operate the brake while doing this or the seal will be damaged.
Author:  67lotus49 [ Sun Apr 15, 2018 12:19 am ]
Post subject:  Re: Front brake master cylinder problem?
Thanks for the reply, going to take it apart tomorrow.
Author:  Ja-Moo [ Sun Apr 15, 2018 12:55 am ]
Post subject:  Re: Front brake master cylinder problem?
And make sure there is a bit of clearance from the lever to piston. Some aftermarket levers are off a bit and don't let the piston return fully. And one other thing that is probably not the cause, but older rubber line can start to come apart inside and the flap clogs the fluid flow.
Author:  Tim Steele [ Sun Apr 15, 2018 7:25 am ]
Post subject:  Re: Front brake master cylinder problem?
Plus one on old hose if the little return hole aint plugged and the calipers aint nasty that is about all that's left.
Author:  67lotus49 [ Sun Apr 15, 2018 3:56 pm ]
Post subject:  Re: Front brake master cylinder problem?
Fixed the brake issue, yes it was the bleed back hole was plugged :thumbup: Also got the Cougar Power valves installed yesterday.
The one bad issue with these motors (RS250's use Suzuki RGV250 motors) is the stock power valves. The stock PV's use roll pins
to hold the 3 piece valves together and the pins will get loose and fall out. Both of my RGV's had pin failures when I bought them.
The Aprilia only was 350 miles one it but cheap insurance for the future. It's amazing in so few miles how much carbon was starting
to build up on them. The new Cougar PV's have the pin part of the casting so nothing to get loose. Plus the add another 6hp. It's
a fun little bike.
Author:  Hollister56 [ Mon Apr 16, 2018 8:18 am ]
Post subject:  Re: Front brake master cylinder problem?
I am having the same problem with my H1.

Is there a bleed hole in the master cylinder some where that could be clogged?
Author:  DGA [ Mon Apr 16, 2018 2:42 pm ]
Post subject:  Re: Front brake master cylinder problem?
Take the cap off the master cylinder, look in the bottom, there will be two small holes, the one closest to the outlet, hose end of the cylinder will be tiny and blocks easily.
Author:  Hollister56 [ Tue Apr 17, 2018 4:19 am ]
Post subject:  Re: Front brake master cylinder problem?
Thanks for the info.

I did find the hole yesterday while looking at the inside of the master cylinder.

It is a tiny hole about 12 to 15 thousandths. Used a very small drill bit to check it. 20 thousandths safety wire was too large.

You can blow air up the exit of the master cylinder to see if it's clear or not.
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