Ajp Brake Question

[squid_on_a_300]

Last week before the first trial of the year I took the bike out for a pre trial shake down and noticed a soft rear brake. The rear brake would not lock the wheel up no matter how much pressure I applied to the pedal. So I bled the brakes and changed the fluid to fresh 5.1. Much better but not exactly where i would like them. On Saturday i was at our club grounds clearing a new section and took the bike around the loop. About 3/4 of the way through the 3.5 mile loop the rear brake locked up tight...i was using a lot of rear brake and may have also been draggin it a bit.. Could not move the rear wheel and had -0- play in the pedal. After it cooled down it unlocked, play returned to the pedal, the rear wheel turned freely and I now have more rear break than before it locked up.

Do AJPs lock up when hot?...I always though brakes faded when you boiled the fluid...

[biffsgasgas]

The play is inbetween the plunger on the pedal at the cylinder? Or are you just moving the lever feeling for play?

Air easily gets trapped at the top of the master cylinder. The best way that I have found to bleed that out is to hang the bike front tire up. Kinda like gutting a cow or pig. The brake switch is also the banjo bolt for the line at the master cylinder. Those switches can leak and it may have been replaced with a true banjo bolt.

Regardless when this thing is standing on end pump the brake pedal, hold pressure on the pedal and crack the banjo there. If you hear air you probably had a pocket up there.

I only mention the pedal play because I have also been fooled by air in the master as play in the pedal. You really have to pull the boot back to look to see if the plunger on the pedal has clearance. I have also found rust in the end of the master cylinder as well.

To answer your question... Yes the caliper can lock up due to heat but why the heat got there to begin with should be your focus.

--Biff

[squid_on_a_300]

Thanks for the reply biff

Play between the pedal and the plunger. Originally I thought it may have been a master cylinder problem due to the fact the play went away and I could not depress the rear brake pedal (until it cooled down and the caliper unlocked) .I know how the heat got there (my dragging the brake)..Just seemed strange to me that it would lock and not fade...i'm going to pull the master and the caliper for a good inspection/cleaning.

[mr neutron]

Added in EDIT: I'm pretty certain they do lock up when hot, especially if you don't have enough free play at the pedal.

If you have enough play at your brake pedal, the caliper is alright, and you manage to get all the air out of the system, I'm wondering if you somehow haven't accidentally gotten something (brake fluid, whatever detergent you use to wash the bike with last, chain oil, or whatever) on the pads? If so, then new pads might be best, but I've gotten lucky with the "Ride around while dragging the rear brake, get it hot, and then quickly hose it down with water" trick. This worked for me.......

Hope ya get it beat!

Jimmie

Edited April 16, 2014 by mr neutron

[lineaway]

I bet while between bleeding and making the brake work you adjusted the brake adjustment. So while riding, the rest of the air bubbled up and wouldn`t you know it the brakes tried to fry! If you did not catch fire. consider yourself lucky!

P.S. Change the pads as it is hard to tell when (They are very small) they are worn out.

Edited April 16, 2014 by lineaway

[thats_a_five]

I have had the rear brake lock up on my Montesa from not having enough play in the pedal. It kept just enough pressure that the pads would drag ever so slightly. It didn't happen at regular slow trials speed but only on high speed transits.

I backed off the adjuster and was able to finish the event, OK. All good when it cooled off. I adjusted a little more free play in the pedal and had no problem since.

[dadof2]

_{Your symptoms are typical of the piston in the master cylinder not returning fully when the pedal is released. This is usually due to a combination of dirt and corrosion in the master cylinder where the push rod joins the piston. These can be hard to dismantle, the last one I did I had to pump the piston out with a grease gun.}

_{When the piston does not return it keeps the fluid pressurised - hence the overheating due to pads rubbing.}

_{The soft pedal is because the non return of the piston means the fill port in the master cylinder remained covered and fresh fluid cant be drawn in.}

_{I prefer DOT 4 to DOT 5.1 DOT 4 is more viscous and a better lubricant. Unless your reservoir cap and handbook say DOT %.1 use DOT 4}

_{DOT 4 is best changed annually on a trials bike.}

[thats_a_five]

Dadof2, Yes, I have had the master cylinder problem you described on a GasGas clutch. The brake problem was on my Montesa and it was related to my efforts to reshape the pedal to not stick so far out and catch on many things. I bend it in and it fits under the clutch housing. I could not adjust it with enough free play and it dragged the pads slightly., I ended up needing to grind down the plunger lever about .100" so I could get more free play with the pedal in the revised (lower) location. Brake pedal is tucked in real close now so no more snagging, bending or breaking of $60 pedals.