Hydraulic bicycle brakes

[Biggles]

Agree URC is trollfest, sad bunch. You might try uk.rec.cycling.moderated but not much activity there recently.

[soup]

Sorry guys I am not really expecting an answer . It's just that I asked in Uk.rec.cycling but it immediately degenerated into a trollfest.

I don't want to go the whole drill out the lever body, tap the hole insert a helicoil reinsert bleed screw route. I think I would rather just pay the £30 for a new lever set-up

Recently bled/burped my (Tektro)brakes but the wee screw that goes into the lever feels like it has stripped. The front lever bleed screw tightens fully but the bleed screw for the rear just spins .

Do I have no option but to buy a new lever for the rear brakes or does someone have a way of rescuing the situation ?

C (or rather the wee hole it fits into).

[Chris Hogg]

PTFE tape on the bleed screw thread?

[RJH]

The diagram lists a bolt for the bleeding port. Can you remove that, see what's going on, and order the part that's failed?

I would err on the side of caution - a lot of pressure in those things.

[Alan]

Er, no, there is no pressure in there unless the lever is pulled. Even then, it isnt much.

[soup]

It's the thread of that hole, so it's the main body,I don't think they supply that separately, Even if they did, I think there would be minimal cost difference between replacing that and buying a whole new lever assembly

I am coming around to that way of thinking myself . "Just buy a new lever assembly and have done".

Think I will try Chris's suggestion of PTFE tape first (have to make sure there are no 'tags' on the tape as the tubes [carrying the hydraulic oil] are very narrow and easily blocked leading to no brakes, ARRGGGHHH!).

[soup]

Agreed it's atmospheric (or thereabouts) with lever relaxed but pressure is massive when the lever is pulled. Force isn't that much, but as the tubes are of such a small cross sectional area the pressure is WAY up there .

Pressures at maximum braking are in the 1,500 PSI region The tubing is designed to work up to the 2,000 PSI region. Atmospheric pressure (depending on altitude and Temperature) is in the 15 PSI region (think standard is 14.7)

[RJH]

Ah OK - looks from that diagram that bolt 'C' screws into the lever, and a non-shown screw is the bleed nipple. But I take it that the bolt is all there is?

Indeed, might be worth a whirl as proof of concept, but there will be a lot of pressure under hard braking.

[RJH]

Um, I did mean when the lever is pulled. It's obviously not much an issue in any event when the brakes aren't being used ;-)

And while not deep sea pressures, I wouldn't trust, say, PTFE tape as mentioned elsewhere. YMMV.

[soup]

Indeed last post there was 14th February. Moderators still active?

[Roger Hayter]

Why not try posting there, it would give them something to talk about, and there is a lot of expertise among the small coterie of contributors?

[soup]

I did, on Saturday it still hasn't appeared. As it takes someone's 'private time' to read and 'moderate' each post . I did not feel it was worth complaining

[RJH]

Agreed - I've had some good advice in the past.

[newshound]

I'd have thought that a Loctite type thread locking compound might have more chance as a quick and dirty fix. Add a jubilee clip around the lot to clamp the screw in place? As tight as possible.

[Paul]

Isn't there a reservoir of fluid in a bladder ?

I thought the bleed would be at atmospheric, the reservoir provides fluid to the master, the master provides mechanical advantage via the ratio of master diameter to slave diameter (trades long throw of brake lever to short throw of hydraulic calipers).

The bleed screw is not meant to be pressurized and is like the plastic lid on the brake fluid reservoir in your car.

I've not been able to find a logical diagram of hydraulic brake operation, nor even an exploded view of the parts inside. Most of the articles seem to plagiarize one another, as poor excuses for adverts.

*******

In this thread, they show a hydraulic brake level, where the side plate (hides the reservoir bladder) has a pinhole. No fluid should be on the inside of that pinhole, and the pinhole is there so the bladder can move and not cause a vacuum inside the covering with the two screws. If the bladder leaked (and the bladder is at ambient, not pressurized), then that's when the space the bladder lives in, would be wetted.

"overhaul the master cylinder... debris blocking the feed port or return port"

And the reservoir is there to feed the feed port or accept fluid from the return port.

One article says the Tektro family uses mineral oil and does not gather water or corrode stuff. And it should have printed near the lever, that it uses mineral oil. Like a car, different brands use different fluids, and the wrong fluid is "death to seals".

It should have a few similarities to a bottle jack, only a really tiny one.

The bleed screw then, needs to "lock", the plastic ring on the screw needs to maintain integrity so fluid does not ooze past. But it really shouldn't "blow" because the reservoir is at ambient. Some "cover screws" (my bike has some for other purposes), they only rotate a partial turn and are a curse (flimsy). Rather than being stripped, I'd inspect to see what type of screw it is. Fully threaded, or just a 90 degree turn for "closure". A kind of "cap" and not a screw.

Paul

[soup]

Hadn't thought of that the pressure in the tube will vary from atmospheric up to 1500 PSI or so. But the pressure on the screw will not vary so much

Try page 5 of this :-

Most are open systems but closed systems do exist but there are not a lot of them

Yes I have mineral oil based fluid other stuff is called DOT something or other

If it was the screw that was stripped I could probably just replace it but it is the actual bleed port but as you say and thinking about it the pressure in the reservoir doesn't vary much. Just tried the brake again and it has held the fluid from yesterday bleed screw/port interface feels only half stripped so maybe it has enough friction (with the half thread and the wee rubber o-ring) to stop fluid oozing out

[David]

might be more helpful.

Cheers

Dave R