Rear brake locking up

[EeeasyTiger]

Got my shovel chopper pretty well dialed in, only thing I can’t figure out is why my rear brake locks up after a couple miles on the freeway. Looks to be a softail caliper. Works well but when I ride on the freeway it heats up and tightens to the point where I can’t move the bike... about to invest in a juice drum if I can’t get this figured out. What do y’all think?

[JBinNC]

Either the relief port in the master cylinder is plugged, or the piston is not clearing it when at rest, or the hose is collapsed on the inside. If the piston(s) are stuck in the caliper from corrosion, that may contribute to the problem, but your symptom is pointing to the master cylinder as the culprit.

Jim

[EeeasyTiger]

Yeah I was thinking the same, it was for drum brakes so I took out the constant pressure (or whatever it’s called) valve out. Any recs on a master cylinder replacement?

[Fetch]

Hacking up the Master was not a good idea. Prior to hanging new parts, I'd take the entire system apart and check it out & clean it out

[EeeasyTiger]

Hacking up the Master was not a good idea. Prior to hanging new parts, I'd take the entire system apart and check it out & clean it out

Since the mc was designed for a drum brake there is no need for residual pressure and it would probably make my issue worse if i had left it in. I’ve done plenty of research and nothing has helped.

Also I’ve had the whole system apart and cleaned and refreshed it

[JBinNC]

The old (cast iron) Wagner master cylinders for the shovels and pans were the same for drum and disc brake systems, except that the residual pressure valve is not used on the disc brake applications. So, you were correct to remove it. The master cylinder you have should work fine, if it's in good order and the pushrod to the pedal is adjusted correctly.

Jim

[DustyDave]

Next time it happens crack the bleeder. If this frees it up it is a hydraulic lock. If it is still locked its overheating from friction and expanding the pads and disc. If its a hydraulic lock get it to happen again and loosen the brake line at the master cylinder if its still locked problem is brake line or caliper. If it frees up problem is master cylinder usually adjusted where it holds the piston partially depressed or swollen rubber. If it's a friction lock correct the alignment or free up the caliper pistons.
Dusty

[EeeasyTiger]

Probably hydrolock. Crack the bleeder and it shoots fluid out as if the pedal were being held.

I’ll have to double check the adjustment on the plunger, never really adjusted it, and then take it out again. Only really seems to happen once I’ve been on the freeway for a bit.

Shouldn’t be any swollen rubbers as I put a rebuild kit in less than a year ago, but anything is possible.

Thanks for the suggestions!

[docmel]

I use a old style wagner pushing my 90's softail rear caliper. Works great. See my 60FL. I used the same setup on a prior build Cheap and works fine

[EeeasyTiger]

Checked the adjustment on the pushrod, it's the late 78-79 style mount where you rotate the plate as far forward or back as needed and apparently when I put everything back together from my frame repair/repaint i had it adjusted as far forward (for the most free play) possible.
Might have to look at replacing the brake hose, not sure of the age of it but I haven't replaced it in my year and a half of ownership. That's about the only thing I can think of.
Correct me if I'm wrong but a collapsed hose would let me push fluid out but not show any signs of moving fluid back into the mc? I remember seeing fluid move when I bleed the brakes...

[Fetch]

Yes, if the line collapses it can hold the pad to the rotor. I re-read your original post, you say this happens after you've been out. If it's free when the bike is cold but locks up after a few miles, it could be the fluid getting too hot/ wrong fluid?

[DustyDave]

If the pushrod is holding the master cylinder off the snap ring any, as the bike heats up it can cause the piston to cover the fill hole which also exhausts the fluid back to the reservoir. In the last 50 years I have seen 2 hoses act like check valves but I've lost count (Several hundred) of the times I have seen the master piston not returning enough to let the caliper exhaust the apply fluid. Never hurts to replace a hose before it starts putting trash in the system. Never hurts to drain the fluid then shine a light in the fill port and watch that the seal retracts enough to open the port repeatedly. If you can't see this one peal the boot back and watch to see if the piston returns to the snap ring repeatedly.
Dusty

[EeeasyTiger]

fetch, i can't verify that if i were to let it sit and get cold that the issue wouldn't still be there because I have to crack the bleeder to be able to move the bike and get it back home when it does. and it was fresh dot 5 so i know i've got the right fluid in there, as to it heating up... not sure about that.

Dusty, I'll have to take a look at the piston return. That seems like a more likely cause.

It's a weird feeling cause I'll be jamming along and then it feels like I'm losing power and i have to open the throttle more and more and that's usually when I realize its the brake thing acting up. By that time it's hard as hell to get the bike to the side of the road and everything is super hot... crack the bleeder fluid squirts out and it's as if nothing happened.

Appreciate the input and I'll report back with any findings. The hose might take longer to report back cause I gotta figure out what mfg made this one and then have the cash to get a new one which probably wont be until next pay day.

[Tater66]

With all due respect to everyone suggestions and respect to your cash flow, I would recommend getting a new master.

I had the same issue on a Panhead with a 4 piston caliper. Replaced with GMA 5/8 bore that has Wagner bolt pattern.
Works like a champ.

[mudmane]

I had exactly the same issue after building my chop. I would get a couple miles from home and my wheel would lock solid. It ended up being the free play on my foot pedal, the brake was dragging just enough to cause it to heat up and seize. I shortened the pushrod for the piston and replaced the fluid and its not caused me any more issues

[EeeasyTiger]

@mudmane yeah I though about that but after I looked into what dusty suggested the rod has plenty of play. Looks like he was correct cause my piston is stuck halfway in the bore. Had a few extra minutes this morning and took a look. Pulled the boot back and nothing was near the snap ring. Pumped the pedal and nothing. Funny cause the pedal felt perfect when I originally set it up and bled it. But my guess is there is a burr either in the bore or something not allowing it to return fully and causing the drag. Time to tear it down, fix it, and ride to confirm.

[EeeasyTiger]

Master rebuilt and no problems since then ( 80 miles or so ).