Help please DR125 front brake disk binding

[paturoa]

Well I'm stumped - help please....

Two rides ago the front brake was under performing, so I stripped and cleaned eveything. But I've done something wrong. For the last ride the brakes were binding and it was un-rideable. So I took out the bleed valve, swapped bikes with the spawn (she rode my bike) and I rode it with no front brake.

Conclusion #1 - I did something wrong.

It is an old DR 125 that I'm getting ready for a camping trip next week.

It is still binding on. I've cleaned (and I mean really cleaned!) the master and slaves, everything in between, and everything looks fine. Seals are fine, groves are clean, no goop anywhere. The slave piston moves freely, the master piston moves freely. I've blown compressed air thru the brake line and that is fine too.

I fill with new fluid and bleed, all as per normal.

Then the problem begins! Arggggghhhhh.

The symptoms are that the lever has pressure immediately, within 1 mm lever pull, and the pads never properly release from the disk. Lifting the front wheel off the ground and it needs serious effort to spin the wheel. When I move the caliper by hand I can see the pads are moving freely on the pins and move back from the disk. I can see that the location of the whole caliper assembly is correct for the disk. But there is immediate lever pressure still and it goes straight back to binding. This includes without spinning the wheel.

So I thought it was the slave end as the master end is so simple that I couldn't have cocked that up. I've tripple checked and the re-assembled. Same problem.

So my thinking yesterday was to start at one end and pregressively eliminate. Starting at the wheel for no particular reason ,other than I'd just pulled that apart 3 times!

- I removed the scraper o-ring from the slave and no difference. Piston nice snug fit with normal "stick-tion" on the seal.
- I blew air back through the brake line and was clear.
- Then pulled apart the master end again (x3) and all good still.

My current thinking is the master is the problem. ... as my understanding of how the brake works is ... that when the lever is released, that the master piston moves backwards (pushed by working spring) to a position where the system goes open to the reservoir. This also has a minor effect of lowerring the pressure in the line and "pulling:" the slave cylinder a small amount. This then releases any pressure and enables the slave to back off the pads and hence the disk.

When I look at it the front seal on the master piston can never reach the reservior hole, well that killed that theory!

So your assembled advise would be much appreciated as I'm out of enthusiasm to pull the sodding thing apart agin as I'm now out of beer and are drinking Mrs P's wine (oh the humanity!)

Thoughts please.

(and the tyre pressure is fine)

[scumdog]

Interior fault in the brake-hose acting like a one-way valve??

It HAS happened.

[paturoa]

Interior fault in the brake-hose acting like a one-way valve??

It HAS happened.

Yeah, I'd thought of that, but not elimated it - wait 1 caller while I reverse it as the banjos are the same at both ends.

Meanwhile, I took the remaining Mrs P's wine down stairs to the garge, and re-assembled the slave without seals and dry.

No issues found there as the disk aligns with about 1 mm to spare, I slide the caliper on the pins easily and using a screw driver moved the other pad up onto the disk.

No alignment issues and the slave piston without seals fits snuggly without wobble and the wheel rotates without grabbing.

Edit: I've been looking closely at the master and the piston does not move far enough back so that the line goes open to the reserviour. The pressure seal stays on the line side of the fluid feed hole from the reserviour. Scratch that understanding of how brakes work!

[paturoa]

Nope - not the brake line - The banjos aren;t the same but I could connect them. Same prob, binding.

[paturoa]

Out of wine, and need to go to get a lot of beauty sleep now, so please expound your theories!

[pete376403]

The seal around the slave piston should be square in cross section. The seal twists slightly as the piston moves out (ie under pressure from the master cylinder) When the pressure is released the seal untwists, retracting the piston thus moving the pads away from the disk. Possibly this seal is getting old and hard, not doing the twist/untwist thing.

[malcy25]

Check the pressure release hole in the mastercylinder. Is a bleed hole just up from the main whole in to the piston area.