Really annoying (and I expect unfixable) problem with Shimano Deore XT brakes

Alright guys, long story short I was commuting on my mountain bike (dark, about 5am) and hit an iron gate someone had left open across the cycle path. The impact was concentrated directly on my rear brake lever, as you can see in this image:
I was traveling at a fair rate of knots, enough to throw me over the bars and unfortunately I believe it has damaged the brake internals. This video: VID attempts to show what my problem is. In the clip, I am applying light pressure to the lever and rolling it back and forth. On the forward movement of the bike, the lever sinks very slightly, and this repeats until it touches the grip. Pumping the lever reverses the effect. As you can see after the first demonstration, applying pressure when the bike is stationary has no adverse effects and this is what confuses me. There are no external leaks, no parts missing.

Anyone got any idea at all what could be the problem? I’m expecting the worst (master cylinder damage) so don’t hold back on your thoughts.

Please excuse the dressing gown and bare feet in the video

so is it actually stopping the back wheel or not?
is there any cable damage?

Under normal stopping pressure yes, it does, but under constant light pressure such as when on a gradient the lever will very gradually sink to the grip. The brakes are hydraulic and there is no damage to the hose.

When were they last bled through? Might be worth bleeding them to make sure there’s no air in the system.

They come pre-bled so I haven’t bled them myself. I sort of ruled out air the the system as I imagine that would just give a spongy feeling rather than this weird behaviour, but it doesn’t seem such a bad idea to be honest.

Who rang?

i would bleed it then see if it continues. failing that and there being no other damage you may have to start replacing stuff.
the fact that it goes in to the bar under light but constant pressure suggests air in the system.

That’s a fault with the master cylinder it’s fairly common and madison a very good at sorting it quickly.
Send it back you usually get a new brake .

That was a phantom phonecall Tom.

Thanks for the input guys, I’m going to bleed them soon as and see how it goes. But nonk, forgive my ignorance, who is Madison?

Shimano importer.

Madison are the UK suppliers/distributors of Shimano…sounds like this is a known issue with the brakes and contacting Madison (normally through your bike shop) will help get this resolved…as suggested, most likely a replacement brake…

Well that sounds promising. However, don’t you reckon they’ll see the damage to the lever and refuse a replacement?

I doubt it, i’ve done more damage when having a shave.

I would still bleed it though (assuming you have the kit), might save the hassle of being brakeless while the faff around with the brake for xx long.

don’t you reckon they’ll see the damage to the lever

I would say it’s pretty likely.

Yeah I have the kit, probably bleed it Friday and if all doesn’t go well I’ll cross everything try for a replacement. Thanks for the input guys.

Sounds like you’ve blown a seal in the MC.
Like when your seals go in your MC on your car, you hold your foot on the pedal and it will sink to the floor as the fluid is passing past the seal. It likely won’t leak externally as its staying in the bore.
If you pull it quickly it’ll still work as the fluid doesn’t have time to pass the seal quicker than is required to move the piston.

As shimano dont do rebuild kits, i’d send it off. Might as well as it’ll need a new lever assembly anyway, unless you can find a suitable seal to fit it.

might just be worth getting a new lever. i replaced one after a crash. cost about £30 i think and easy to do

I’d not use a brake with that much damage.
A scratch on the lever like that could be fatal.

That was my thought mrjmt, if a bleed fails I’ll get it sent. The only confusing part however is the fact that it doesn’t happen when the rear wheel is stationary.

Oi less of the sarcey, stu, it looks like a scratch but it’s actually quite a deep indentation. I was trying to show how hard the lever hit the gate.

If you’re going to replace a lever you might as well do the whole master cylinder assembly. I got one for about £35 on CRC I think. Keep the old one for spares

You guys realise that distributors read these forums too?

I had an SLX rear brake replaced after it started to become unreliable. I would pick up the bike after a few days of not riding it, and the lever would hit the grip. A bit of pumping would give me some braking force but I was having to sod around with it before every ride. I bled it 3 or 4 times, but to no avail.

I sent the brake back and just got a new one, which thus far has been fine.