Yamaha TRX850 Enthusiasts Forum

Hey guys, got a bit of a problem with rear brake drag on my 99 trex (with 37,000 km). The rotor gets very hot (too hot too touch) after a ride, even if i hardly use the back brake. The pads have also almost worn out in the space of maybe 5000 kays. A couple of months ago i completely pulled the rear caliper apart, gave it a good cleaning with brake cleaner and a toothbrush and put new piston seals in it. I also gave the pistons and seals a coating with the grease that the seals came with (i assume silicon grease). This helped a little bit but the rear brake still drags a lot more then the fronts. Any ideas guys? Thanks

one or both of the pistons could be knackered, out of round, ridged etc

crap behind the seals I would have thought

i'd be surprised if the rear disc hasn't warped as well which will not help matters

Its definitely not crap behind the seals/in the caliper, its spotless inside. Is there anything in the master cylinder that could prevent the pads retracting? I gave it a good clean out with brake cleaner but didn't pull it apart. It was a bitch to get primed from dry, but that seems pretty normal, yeah? Thanks for your help so far guys keep it coming!

possibly whatever you've used to clean the caliper has made the seals swell.

Oh and i can't detect any vibrations when i apply the back brake that would make me think its warped, though i'm sure it could be a little warped without being noticable to me. The rotor is a bit scored, however. I think its been like this since before i bought the bike (a year and about 7,000 km ago)

Just pretty standard spray on brake cleaner... Could the silicon grease have been a bad idea? I thought you were supposed to use it on the piston seals?

If the brake's dragging, its gotta be something not 'returning' to where it should. Check the whole system, starting from the lever to see if it's returning correctly, right through to the pads. Are the pads the right ones? Are you riding with your boot catching the brake lever (it does happen!) but most likely a piston issue like Youngy n Phuk say.

I thought it was copper grease for the back of the pads/pistons? They get damn hot.

crap behind the seals I would have thought

...how often does he need to do that ??

Pretty sure the pads are factory yamaha, i do tend to use the back brake more then a lot of sports bike riders do (its so powerful-almost too powerful) but i definitely don't ride it like a cruiser (ie lots of back brake). I don't think its my foot, but i will keep it at the forefront of my mind when i go for a ride next and make sure my foot isnt anywhere near the lever. I noticed the rod that goes into the bottom of the m/c looks like it has some kind of threaded adjusters on it. Whats the adjustment for, any connection to the dragging?

If my five eggs are worth anything, I'd have ONLY used brake fluid as both cleaner and a lubricant... can't possibly go wrong then!

G

I have to agree Greg. I have used brake cleaner and metal parts cleaner on brakes and had similar problems. Both of these products were sold as suitable for the job. The only grease I would use on the seals is red rubber grease, not silicon grease.

if you replaced all the orings / seals and everything is spotless then your problem lies with the brake cleaner. (as has been said, I wouldn't use the stuff personally but .. )

If it wasn't completely dried up or removed, it could have reacted with the barke fluid and caused the seals to swell.

I'd be inclined to throw it away and try and find a working one (often cheaper than new seals / pads!)

Also chech that you have some end-float on the actuating spindle at the master cylinder. You adjust it by loosening the 10mm AF locknut and rotating the spindle. You may have to remove and rotate the clevis pin to readjust the brake pedal height.

So by end float you mean a little bit of up and down free movement (freeplay)?

Also make sure your master cylinder is free to retract when your foot comes off the pedal.
It should spring back to the off position under its own spring tension. Linkages and pivots should be free. You could lift the back wheel up apply brake and if its still binding crack the brake hose if it frees up your problem is master cylinder related if it is still binding your caliper pistons are sticking. One last thing do not overfil, leave some air space in the resivoir.