Left (back) brake handle Still spongy after MULTIPLE bleeds

So earlier in the year i had a leaking right brake master cylinder handle so i replaced BOTH cylinders just to make things even. the new right brake is WONDERFUL, super stiff and a light press activates brake light and stops the bike. but this left  one even being brand new was always a little issue. when i bleed it, it acts Great for the FIRST day or maybe 2 days. Then it gets spongy and sticky with a BIG deadzone in the handle. (this gap the pump isnt even touching the handle) and because the handle is "in the air" its making the brake light stay on unless i push the handle forward. and also the left cylinder brake fluid gets REALLY dirty like it was exposed air or something. Now the old factory LEFT cylinder wasnt broken but i just replaced both to be safe (the handle was bent) the nest thing ill try is to put the old one back or take out the pump from it and try that. its a new cylinder so i dont know whats the issue, the line, the pump or caliper? i doubt the caliper is bad because when i changed the brake pads last year it was food, and even now despite how spongy the back break is it still stops on a dime.

Strange it should get dirty so quickly, maybe a rubber brake line deteriorating?

oldwingguy wrote:

Strange it should get dirty so quickly, maybe a rubber brake line deteriorating?

And on top of that ONLY the left gets dirty, the right line that is tight is as clean as the day i put it in a couple months ago.
correct me if im wrong but aren't both Lines connected in a way, when you press one brake it slightly presses the other one too no?

The left brake activates the rear brake caliper and only one piston (of 3) in the front caliper —thereby linking the front & rear brakes. The right brake activates only the other two front caliper pistons. So the right brake isn’t linked to the rear brake. There is only one piston in the rear caliper.

I made the same error once rear caliper has 2 pistons but has nothing to due with OP's problem.

Sound like left brake master is bad maybe seals are torn up or rear caliper is sticking and overheating fluid. Lever is sticking bad master cylinder most likely.

ill test that by installing back the old working master cylinder and see how it acts

As the OP wasn't too sure how the brakes are plumbed up I'd guess its just a bleeding issue, they do need to be done in the correct sequence.

I had long relationship with this problem since I overhauled the rear caliper and the rear master cylinder followed by problems bleeding etc.

After replacing almost every part of the whole braking system, (slight exaggeration), and still having the same, problem it came down to a bad rear master cylinder and the rear caliper pistons getting 'sticky'.

Thinking back on it now, I was never completely happy with the rear caliper because of powder coating overspill near the bores, which I cleaned away. Also, I think I used the cheapest master cylinder kit money could buy; bad move.

Just my two bobs' worth.

Also one thing I forgot to mention. After riding and pumping that brake for like 10 minute it stiffs up nice and tight again like factory. Come back to it tomorrow. It lost all the preaure.

Well that's curious. During my dilemma, forum member 'tech 1 twa' suggested pulling the rear brake lever very slowly and if it comes back to the bars without resistance, then the master cylinder is the culprit. This turned out to be the case with after all my work.

If this is not the culprit then I would say it's 'air in the system'....somewhere. I let the shop do my bleeding, I guess I don't have 'the knack', despite several attempts.

Loosemarbles wrote:

Well that's curious. During my dilemma, forum member 'tech 1 twa' suggested pulling the rear brake lever very slowly and if it comes back to the bars without resistance, then the master cylinder is the culprit. This turned out to be the case with after all my work.

If this is not the culprit then I would say it's 'air in the system'....somewhere. I let the shop do my bleeding, I guess I don't have 'the knack', despite several attempts.

Should I bleed BOTH cylinders even though I'm ONLY having issues with the left? Because I have not been draining both. I did the right one ONE time and it was tight and good. I though theirs no need to drain the right one seeing how it was great.

Yes, bleed BOTH valves for the rear brake. Bleed the rear brake linked hose on the front caliper first, then the rear caliper.

XDM, are you bleeding the rear cylinder through the nipple on the front caliper as well as the rear caliper?

In theory, you shouldn't need to bleed the front cylinder at all if that brake is good. It's the operation of the front brake from the rear lever which is in question, i.e. that second nipple on the front caliper.

From our forum back when.

A. Front = right hand reservoir = upper front caliper bleeder valve.

B. Rear = LEFT hand reservoir = FRONT LOWER bleed valve THEN rear caliper bleed valve.

I have a folder that I keep with tips and hints from here, print them out and keep them handy.

Please forgive me for sounding presumptuous but I suspect that XDM is wondering if the right brake lever could affect the effectiveness on the rear brake lever, affectively speaking! On my non-abs SW the answer would be no. Don't know about ABS systems though.

So far my bike likes to make me look like a fool, for 2 days SO FAR it kept pressure but I'm still going to take action with it very soon because it WILL happen and I want to find out why.

I know how that feels.