mk111 sticky rear disc brake

Status
Not open for further replies.
W

worntorn

VIP MEMBER
Joined
Dec 22, 2006
Messages
8,193
With + 50 f temps and sunshine I took the Commando out for a 50 mile ride this week, first usage in a year or so since lacing up new wheels for it.
The rear brake started to drag on the way back and eventually got very hot. I had to stop and let it cool down a couple of miles from home, then limped in slowly. After letting it cool I removed the caliper to see if the pads had been ruined. They had, one lining fell off the pad as it came out of the caliper. Putting new pads in, I noticed the pistons move back quite smoothly and freely, doesn't seem like they should stick, but they still are with new pads in. When I crack the bleeder, a small amount of brake fluid escapes and the brakes stop dragging. Maybe there is some air trapped in the system?
Gotta love that MK3 wheel quick detach/ cush hub design, it sure makes removing the wheel easy.

Also, my caliper bracket has a serrated face copper washer between the chain adjuster and the bracket plus a corresponding thickness steel washer between the bracket and swing arm at the shock mount/caliper mount location.
This keeps the bracket in plane with the swing arm, but neither washer is shown in the parts book. The caliper is just a bit off centre on the disc, 20 thou removed from both washers would centre it up. Removal of the washers would put the caliper about 40 thou off centre. Are these washers found on other MK111s?

Thanks
Glen

mk111 sticky rear disc brake[
 
Hi Glen, Could the pedal adjustment be limiting the piston return, perhaps the spring in the master cylinder isn't working properly or the piston isn't free enough. I also had to muck around with the caliper position on mine too, had to bend the bracket and put a washer between the caliper and plate to get the disc in the middle and lined up. A quick check for me is to see if the caliper moves (and it shouldn't) when the brake is applied. I guess the pads would wear in eventually but I thought to get it as close as possible anyway.

Jeff.
 
worntorn said:
When I crack the bleeder, a small amount of brake fluid escapes and the brakes stop dragging.
Click to expand...

Once the brake has been released, any residual pressure in the system should bleed off into the reservoir through the vent port, for whatever reason this doesn't appear to be happening.

How long since the rear master cylinder was overhauled?

http://www.classicbike.biz/Triumph/Main ... Norton.pdf

How old is the brake hose and is it the 'rubber' type?

http://www.hoseandfittingsetc.com/techn ... ake-lines/



worntorn said:
Also, my caliper bracket has a serrated face copper washer between the chain adjuster and the bracket plus a corresponding thickness steel washer between the bracket and swing arm at the shock mount/caliper mount location.

Are these washers found on other MK111s?
Click to expand...

Certainly not my MkIII.
 
Les, the rear master cylinder was rebuilt in 2000. The brake hose is the rubber type. It is at least ten years old. No visible signs of deterioration in the outer rubber sheath though.

Glen
 
worntorn said:
Les, the rear master cylinder was rebuilt in 2000.
Click to expand...

How long since the fluid was changed ?
Its recommended quite often...

I've seen a small residue of dust looking stuff, in the fluid, that can block or limit that little fluid return hole.
 
Fluid was changed a few months ago.
Reading thru the info Les posted, the brake hose seems a likely suspect. The instructions state that an internal rubber flap in an old hose can act like a one way valve and cause the brakes to drag intermittently. Since the hose is well past the suggested renewal period (every 3 years!) , I will replace it and see if the problem goes away.

Glen
 
Glen the first thing I would do is put 'er up on the center stand and get down low on a comfy floor pad and visualize up the caliper then start spinning the R.wheel. It should be apparent quickly the accuracy of trueness. Then all of the above good advice. :|
 
I just replaced the rubber lines on my bike because they had colapsed on the inside restricting fluild return. try blowing through it manually not with compressed air and see what happens .......

Tim_S
 
Besides collapsed hose and cable return not quite uncovering the tiny pressure bleed off port its possbile the plunger pressing on the master cylinder piston is a tad too long with new pad thickness to fully uncover bleed port. Too short a plunger/spacer usually only shows up as brake heated so might try hair/hot air gun on caliper to see if heat inducing drag for a clue or maybe a cold glass of water on it after a bit of test ride to see if relieves the drag.
 
Rearsets fitted? I rode with a guy who inadvertently smoked the rear on a borrowed bike for (incorrect) ergonomic reasons. :idea:
 
It's a stocker, no rearsets.
Steve, the sticking started with the old pads in.

Glen
 
Ugh Glen I'm not equipped very well mentally mechanically but to stumble-fumble Commando level issues so hope I don't run into brake drag on Peel project and you find a simple thing to alter and leave wonder ride tales a long time. I don't know if pad thickness affects the piston returning far enough to uncover the bleed hole so still up in the air factor that took me down on Peel 50 mile into 2000 mile 1st time ever long trip.
 
Ok LAB, then strike off pad wear issues in Glens heated dragging. No danger to shorten the parts pushing on piston by a few 100th's to allow a tad more piston spring hole uncovering return except shortens effective piston pressure travel by that much. If air layer still present its heating may also develop some pressure on pads but is also felt as brake fade absorbing lever pressure when actually braking.
 
hobot said:
No danger to shorten the parts pushing on piston by a few 100th's to allow a tad more piston spring hole uncovering return except shortens effective piston pressure travel by that much.
Click to expand...

Well, we don't-as yet, know for certain that this problem is being caused by the master cylinder piston not withdrawing sufficiently for the seal to uncover the vent.
Also, the usual way to adjust the pushrod so the piston uncovers the vent is to follow the procedure given in the Lockheed Brake Service manual, (sheet 114B, 1 - 5) rather than by shortening the pushrod.
 
Ahh, the old Lockheed Brake Service Manual Sheet 114B, 1-5, I should have read that before I started sorting out my brakes......... but noooo, just blundered on in, fitted those pesky stainless pucks, tried several different pad compounds, used non standard braided hoses, DOT 5 instead of DOT 4 fluid, (it's all I had) adjusted the caliper to get it aligned perfectly with the disc, made every mistake possible, now the rotten thing locks up the AM26 if I push the pedal too hard. You would think after 40 years of aircraft engineering I would have learned to read the manual first.
Couldn't find any reference to the 1/2" sleeve down kits for the front master cylinder in that manual, can anyone tell me what section that's in please?






I have been called cynical before!
 
Les,
Any chance on getting a link to that Lockheed Service manual information?

Worntorn,

Something happened to my MKIII after it sat for a while. I bit the bullet and rebuilt everything. Pay close attention when you disassemble the master cylinder with respects to how far you install the actual cylinder into the body. Count the turns.

https://www.oldbritts.com/14_064244r.html

I also found it easier to bleed the whole system off the bike and then install it back on in one piece, keeping the caliper above the master cylinder. I used Ferodo Platinum pads and SS pistons.

I was also able to take my old rubber hose to a local hydraulic shop and have them make one up.



Also I find Hobot's trick of poking a hole in the seal interesting. Wish I had known about it before it did mine

Good luck,
Pete
 
Status
Not open for further replies.

Similar threads

M
F/ Disc brake hub centralised?
2
Replies
27
Views
1,325
ludwig
ludwig
baz
  • Locked
Rear disc brake conversion
Replies
7
Views
1,002
Danno
Danno
Denis J
  • Locked
Fitting a rear disc brake on a drum brake Commando
Replies
19
Views
2,068
ludwig
ludwig
T
  • Locked
Front and rear disc brakes on same side.
Replies
14
Views
1,680
triumph2
triumph2
freefly103
  • Locked
Drum to Disc brake (2019)
2 3
Replies
52
Views
8,826
Time Warp
Time Warp
Back
Top