Rear brakes drag when hot.

[Green Mtn venture]

Hi everyone. I am new to this forum and just got a 1983 VR. It's a project bike. Been sitting for a couple of years before I got it. I got it to run ok but when I take it for a short ride the rear brakes get hot and drag. When it's cold they work fine. Any ideas? I have a full peddle. To me it seems like the Caliber might be dragging a little bit as it gets hot everything expands and binds up. Thanks for any help you have.

[Snaggletooth]

Yep, when the brake fluid heats up it expands and being the shortest line to the rear brake it causes the pistons in the caliper to drag a bit, sometimes to a full lock.

 

Use an 8mm wrench and crack the rear caliper bleeder and drain off a bit of brake fluid. Not much, just a couple short pumps.

 

That should clear that problem up for you.

[Squidley]

Your rear master cylinder and reservoir are full of crud from the brake fluid breaking down. Your going to have to tear all of it down, clean out all the crud inside and make sure the return orifice is clean. What is happening is that the fluid is expanding when it gets hot, it's in a lousy spot on the bike. The return orifice is plugged and wont allow the expanding fluid to freely flow back up into the reservoir. The pressure has to go somewhere and thats the caliper, so it locks up. Very common on bikes that have sat for a while and not had their fluids changed often enough. Give a call if you have questions, I can walk you through it.

[Kirby]

Another thing to look for is air in the line which will expand as the brakes warm up and cause the pistons to close on the caliper. If this bike has been sitting for as long ad you say, then the brake fluid probably needs changing anyway. Best to drain and inspect then replace and bleed. Let us know how it works out. Someone's always willing to help here.

[Green Mtn venture]

Thanks guys. I'll take a look at it tomorrow night. I did notice the the brake hose that goes up from the peddle to the plunger is cracked so that will need to be replaced anyway. I think once I work out all the bugs ill really like the bike.

[MiCarl]

There is a small orifice in the master cylinder that releases that pressure when the brake isn't applied. Some things that will cause the pressure to not release:

 

1) Orifice is plugged up with crud from old, crusty brake fluid.

2) Reservoir is overfilled so there is no place for the fluid to bleed off to.

3) Not enough free play in the pedal linkage. Piston does not retract enough to uncover the orifice.

[Condor]

There is a small orifice in the master cylinder that releases that pressure when the brake isn't applied. Some things that will cause the pressure to not release:

 

1) Orifice is plugged up with crud from old, crusty brake fluid.

2) Reservoir is overfilled so there is no place for the fluid to bleed off to.

3) Not enough free play in the pedal linkage. Piston does not retract enough to uncover the orifice.

 

 

And the expansion hole is under the plastic elbow on the side of the master cylinder. It's held in by a philips screw, and an 'o' ring. So it'll resist a bit before coming out. In the bottom of the hole there are two smaller holes. One looks like someone started to drill and then changed their mind. At the bottom of that hole is a micro hole that lets expanded DOT 3 back into the reserve. It can be cleaned out using one of those wire ties with the cover stripped off. Once you get it clean bleed the rear brake. If you don't have them get a set of 4 ea. 8125L Speed Bleeders. Worth every penny. You should be good to go. I had the same problem with my '83 when I first bought it. I fought that SOB for the better part of a month, even after rebuilding everything, until... I discovered the expansion hole and put two and two together and reasoned what it was for.

[icebrrg3rd]

Like they've all said, clean out the master cylinder and definitely clean out the expansion hole. I dealt with the locking rear brake on my first long run (never got it hot enough locally) and just did the bleed to release the caliper to keep moving on the trip. Once home, I tore it a part and cleaned it well, new fluid, and haven't had a problem with it since.

 

It's not that hard, and other than new fluid, I reused all my old parts as they were in fairly good condition.

 

-Andrew

[Guest bodasefus]

I had that happen to me while doing 75 mph on the interstate,,, I smelled something burning or hot... when I disengaged the cruise control,,, I started to slow down immediately... I got over to the edge of the road... I opened the zirk and bled out each caliper and was able to get it moving enough to get off the interstate... then AAA towed me to my mechanic...

They cleaned out the rear reserve tank and the tiny holes in the bottom... A total flush of the system and cleaned up the calipers...

I am going down today to pick it up and ride home...