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Hi, I've asked about brakes previously, but on a different area.
Bike is a 1993 GPZ900R, well sorted. Just back from a trip around the Dolomites and Swiss Alps, bike was great. But a query on the front brakes.Â
The calipers were completely stripped, ultrasonically cleaned, replacement seals etc 12k miles ago and since, I've kept them very clean. The reservoir is original and runs 2 brake hoses, one directly to each calliper. Before this trip, I carefully bled the brakes as a precautionary measure, but also to try and resolve this symptom...
When I do frequent heavy braking e.g. descending a mountain pass with multiple hairpins, the brake lever bite point increasingly diminishes (irrespective of brake lever setting). So at the start, you maybe pull the lever e.g. 20% before feeling the bite start, and then continue to squeeze to a very solid stop. After a series of hairpins, the 20% falls to 2%.Â
At no point are the brakes spongy, cold or hot. The reservoir is correctly filled with plenty of airspace. The fluid is fresh. Bleeding pre trip produced no air bubbles (I've lots of experience bleeding brakes). There's no brake drag apart from the descent of St Bernard where the brakes effectively gently seized closed on the calipers - I then removed the brake lever, allowed the master cylinder piston to 'pop' a little, and all was then well.
I'm assuming there's a pressure build up from heat? Master cylinder sticking? And accept they're 30yr old brakes so maybe just how it was then! And they work remarkably well outside of numerous hairpins.
Any clues? Would love to understand.
Cheers
Is the brake reservoir a closed system? Ie without a rubber expansion bladder?
It might just be expansion = no room for the brake fluid to expand therefore the biting point is reduced?
The reservoir has a rubber expansion bladder...
The reservoir is c. half filled with fluid (there's a sight glass to check level).
Presumably if the bladder was leaking, then pressure wouldn't build up?
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Sounds to me like the port from the reservoir down into the master cylinder isn't clear. When the lever is released & in the resting position there should be a clear path from the reservoir down into the hoses and calipers. I'd strip the MC & check the piston seal is clearing the port.
^that was my next 'port' of call!
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Does the brake bleed okay old school method wise? - ie bleed nipple open/brake lever in, close and repeat
Is the brake lever original to the bike/reservoir? Can the brake lever be pushed away from the grip/handlebar? - my thinking as per Dave above is that the reservoir port is closed off, possibly due to a non original lever blade or something to do with the bite point adjuster pushing the master cylinder piston too far into the bore and keeping the port closed.
Two good answers there. That was my suspicion as well.Â
I recall my a8 did that once when I went from the stock 3 hose setup to a braided 2 hose. Had all sorts of issues. Bristol Kawasaki reckoned it was because of the anti dive !
Thanks all. Ok, so it's a aftermarket lever, a good quality one,. It easily pulls away from the master cylinder. When cold, there's more travel to reach the bite point than I'd like, which was the same with the OE lever. I have moved to 2 individual braided hoses and fitted riser bars.
I'll check out Dave's suggestion of the m/c port seal sticking, that instinctively makes sense. Presumably if I buy a m/c service kit, then that's a simple way of checking?
Much appreciate the input, thanks
And it bleeds very nicely via the bleed nipple.
Thanks all. Ok, so it's a aftermarket lever, a good quality one,. It easily pulls away from the master cylinder. When cold, there's more travel to reach the bite point than I'd like, which was the same with the OE lever. I have moved to 2 individual braided hoses and fitted riser bars.
I'll check out Dave's suggestion of the m/c port seal sticking, that instinctively makes sense. Presumably if I buy a m/c service kit, then that's a simple way of checking?
Much appreciate the input, thanks
And there's my experience. Aftermarket lever. Had it twice in 20 years but the lever was just slightly binding the brakes as the fluid overheats it expanded.Â
They may look correct but it only has to be very very slightly differentÂ
Is it green? If not that's probably your issue right there!
Ok, thanks again. Will service the m/c first and see if any difference and then look at picking up a OE lever...thanks