The rear brake on my 06 4rt rear is fine at first but after a few minutes riding starts to tighten and starts to bind, eventually seizing on. At this point there is no free play at all on the rear pedal, it's completely tight. 

At this point I either wait for it to cool down and release or release some fluid out of the bleed nipple. Any ideas on what is causing it?

You have the adjustment in too far on the master cylinder. There is a hole between the master cylinder and the reservoir that opens when the brake is fully off. This allows the system to depressurize. When you actuate the brake the piston in the master cylinder moves past the hole so the pressure builds up in the system and actuates the pistons in the caliper. If the adjustment is in too far the hole is never uncovered so the brakes can drag a little, build up heat in the fluid which then expands causing more drag causing more heat continuing until the brakes lock up.

The good news is the adjustment is easy. Just make sure the piston in the master cylinder returns all the way bottoming out on the circlip.

or your disk could be slightly bent

I would check the adjustment as indicated by Dan and if that doesn't show a problem or doesn't fix it then investigate the possible bent disk as Nigel suggests, and don't overlook a possible issue with the caliper as well.  

9 hours ago, dan williams said:

You have the adjustment in too far on the master cylinder. There is a hole between the master cylinder and the reservoir that opens when the brake is fully off. This allows the system to depressurize. When you actuate the brake the piston in the master cylinder moves past the hole so the pressure builds up in the system and actuates the pistons in the caliper. If the adjustment is in too far the hole is never uncovered so the brakes can drag a little, build up heat in the fluid which then expands causing more drag causing more heat continuing until the brakes lock up.

The good news is the adjustment is easy. Just make sure the piston in the master cylinder returns all the way bottoming out on the circlip.

I had exactly this problem on my front brake when I first bought the bike adjusted the brake too tight. Inevitably it threw me off within two minutes of starting riding. 

Disk brakes are inherently self-adjusting to compensate for disk and pad wear by allowing the pistons in the caliper to have a range of motion which is far greater than their actual working travel. The throw of the pads is very small when actually braking. The majority of the restoring force, the force that pulls the caliper pistons back, comes from the Viton o-rings that seal the pistons in the caliper bores. In a properly working caliper the o-rings will only flex so far before slipping on the outside of the pistons. For example when you rebuild a caliper or even just push the pistons in to change pads you are sliding the o-rings to a new rest position on the piston. When you put the caliper back on the disk you have to keep pumping the brake lever/pedal to push the pistons out far enough to get the o-rings to slide so if it’s working correctly, each pump of the lever slides the o-ring a tiny bit on the piston until you have moved it enough for the piston/pads to be within the working distance of the brake. Once there additional actuations of the lever move the pads out until they contact the disk and can’t displace any further. Since the travel of the pistons is now limited by contact with the disk the o-rings are within their elastic limits and stretch without sliding on the piston. The self adjust is done and now the brake works as intended. When you think about it it’s quite clever.

What happens with a bent disk is the disk can force the pistons back into their bores outside of the working distance of the brake. This starts the self adjust cycle again which is why you will have no brakes but can pump them up until they work but as you ride the bent disk pushes the pads back and again you need to pump the lever to get the pads back to working distance. So a bent disk will manifest as almost the opposite of what you are experiencing.

On my Beta 4t it was the small O ring in between the two caliper halves that had degraded and wouldn't allow the fluid to return after braking.

I had a similar problem on my 4RT. I’d over filled the reservoir which put the brake on when the fluid got hot from the exhaust. Probably the same effect as over adjusting as discribed by Dan. 

The reseviors on the Beta have rubber bladders to prevent overfilling so there’s always space for fluid expansion. I don’t know if the Montesa is the same.

I had this problem on my Gas Gas.  I found the rear pads were almost worn out and making the piston come out too far in the caliper and causing the rear brake to bind slightly.

If the rear pads appear low, I would change those first.  Then check the stuff above if that doesn't cure it.

Hope this helps.

 

Bruce.

Ditto what brucey says, as the pistons come out they have less tendency to return as easily

This can also happen just from air in the line. For similar reasons as when you've worn the pads too much, with trapped air the pistons can stop retracting, which means the pads drag more, which generates more heat, which makes the problem worse. 

If the pads look OK and the pedal is adjusted properly, with free play, make sure there's no air in the system. 

Edited November 17, 20178 yr by heffergm