I believe I may know the answer to my question after searching this board for a bit but thought I'd make sure. I believe I >may< have a dual disc master cylinder. I have a four piston GMA ( I think) single caliper and an aftermarket master cylinder. My problem is that the brake lever has little, or no, travel and is always nearly fully pumped up. The front brake works fine, just that the lever travel varies from almost none (brake light stays lit) to just enough to turn off the brake light.

Dual disc master? Or something else? If it is a dual disc master cylinder, is there anything I can do to help it to "lose" some pressure and avoid having to buy a new master?

Thanks,
Rich

 

now to be honest if you say the brakes work fine then its not a dual disc master cylinder Brakes are one of the most important things saftey wise next to your tyres and anything done to lower pressure would only be asking for the whole system to fail. Here is a few things to check.
Has it always been like this since you fitted the parts/bought the bike.
Not knowing which yearor modle bike your on doesnt help.
If its only the brake light check to make sure the switch is working corectly. Do you have the stock controls with the brake switch in the housing or does your aftermarket master cylinder have its own switch or does it use a pressure operated one in the brake line?
If its the harley one make sure the mushroom shaped head is okay and not damaged as they can stick.

Also make sure the brake lever isnt touching the end of the pushrod in the master cylinder, is there an adjuster for the free play? or you might just need to shave a few thousands off the end of the lever where it touches the end of the master cylinder piston. The lever should not push on the end of the piston all the time, you should have a small amount of free play which is what lets the fluid drain back into the resevior.
What fluid are you using. Dot 4 or dot5.1 will give a very firm feel to the lever where as dot 5 even when perfect has a spongee feel to it.
On the caliper front make sure the piston and pads move freely. The pistons might be sticking and not letting the pads return hench the pumped up feeling.

In reality what your describing is a real fully working effective brake system, its only harley who sell bikes with brake systems that have so much slop in the lever and no real feel for how much pressure your applying. Next time your out drop by your local metirc delaership and try the brakes levers on a few of the fast jap stuff, thos ebrakes can bring a bike to rest from speeds harleys only dream of with perfect control without drama.
ANy more questions feel free to ask or PM me.

 

The bike is a 95" FXD convertible that came with dual front discs. Years ago I changed the front end (wide glide trees, heritage legs, Revtech single four piston caliper w/SS line) and changed the OEM bar controls to something from Drag (if I remember). I believe I used DOT 5 fluid. Now, I don't remember exactly why I decided to change the master back to the OEM a couple of years ago (I'm not young any longer and my memory says it may have been that I kept having to bleed the front brakes too often??) but ever since I did I've had the problem I described. Previous to this spring the brake lever was always pretty hard and had very little travel, but it seems the more I rode and the warmer it got outside the problem would lessen and I'd have a bit more travel and it became less of an issue. This spring however, the lever is really hard and has almost no travel. What I'd done in the past was to brake fairly hard a few times and I'd get some travel back, but so far that's not working. I pulled the Drag master cylinder apart this morning and measure the piston to be 5/8". I'm guessing my OEM right now is 3/4"? I could get a rebuild kit for the 5/8", reinstall, and hope it bleeds better than it did. I see there are 9/16" masters available, but I sure don't want to toss bucks at that solution.

The brake light tip is one I'll need to check on this aft if I get time.

Thanks!

Rich

 

okay thats much clearer. So now your saying your not running the aftermarket master cylinder but the orignal one form the dual disc set up the bike came with.
Well that why you have hardly any movement, the orginal set up was twin single psiton floating callipers. Now you have a none floating 4 pot gmc the master cylinder is trying to flow enough fluid to operate the old calipers which is way to much for the new single one. I have arleady gone on at length in another post about how hydrulic brakes work.
So how I read it you swaped the master cylinder back several years ago, did you drain all the fluid out when you did the swap ? sounds like you have water sat in the caliper, when you use the brakes it creates a vapor buble in the fluid which is what give syou the travel back. First step is to totaly drain the sytem down and flush it thorugh with brand new dot5
If you were having to constatnly bleed the old sytem you migh have had a bad seal on the banjo bolt on the master cylinder. Rebuild your drag one and try thay. |Or see about picking up a later single disc OEM off soemone on here who has upgraded or gone the custom route on there ride.

 

I'll probably rebuild the 5/8" Drag master cylinder and reinstall and hopefully get it bled good. This would be the cheapest route. Lots more tips available now than 10 or 12 years ago on how to bleed.

Thanks for the tips.

Rich