locked REAR disc

Apologies if this has been covered but I couldn't find anything.
Just returned from a approx 30 mile ride on my recently completed Mk3. I had previously had problems with my front brake locking up after the bike sitting in the hot W.A. sun. I replaced the sleeved original master cyl with one of the new units from Andover. Great result! Plenty of power and feel and no locking up.
When passing through town this afternoon I experienced a very similar problem with the rear brake after stopping to get a drink. When I rolled the bike off the stand and used the rear brake it locked.
It is quite hot today - max 35 degrees C.
Master cyl is a S/S unit from Rocky Point - seemed good quality etc and no problem till now.
Has anyone had this experience with S/S rear cylinders? I am not sure how to proceed from here. I would have thought it would have been OK.
The front brake was black and in the sun. It used to get hot to the touch and was an assembly of very different materials (with subsequently different co-efficients of thermal expansion) and so I was not so surprised. The rear cyl is more protected and (from memory) comprised the S/S barrel and S/S piston.
Any advice from you bunch of wise heads very appreciated.
While I think of it - a very merry and rewarding Christmas to you all.

SB

Bleed it again, check that piston in M/cyl is retracting freely.

Open the bleed valve on the caliper first, and see that releasing the pressure does or doesn't back off the brake pads....

shedweller said:

Has anyone had this experience with S/S rear cylinders?

Click to expand...

It's a reasonably well known MkIII rear brake problem which does not just affect the S/S cylinders, the usual cause is the master cylinder barrel assembly screwed too far into the housing, the result being that the M/C piston does not retract far enough to open the bleed port to the reservoir when the brake is released, consequently any excess pressure remains trapped in the brake line and caliper so the brake drags, or in extreme cases, lock up.

I suggest you remove the barrel assembly and re-install it according to the Lockheed instructions: http://www.classicbike.biz/Triumph/Main ... Norton.pdf

I had a similar thing happen to the back brake on my T140 on a hot day - turned out that the hose was old and blocked up and wouldn't let the pressure off.

Mr Bean said:

I had a similar thing happen to the back brake on my T140 on a hot day - turned out that the hose was old and blocked up and wouldn't let the pressure off.

Click to expand...

Yes, old rubber brake hoses can collapse internally and cause the same problem. Rubber hoses should be replaced at least every 10 years, if not sooner, hopefully there's nobody out there who still has the original brake hose/s on their Commando? :shock:

http://www.hoseandfittingsetc.com/techn ... ake-lines/
http://www.braidedhoses.com.au/whyreplace.html

L.A.B. said:

shedweller said:

Has anyone had this experience with S/S rear cylinders?

Click to expand...

Click to expand...

, the usual cause is the master cylinder barrel assembly screwed too far into the housing,


Sorry Les,
It does it means the cylinder is too screwed or less?
Regards.
Ciao
Piero

pierodn said:

It does it means the cylinder is too screwed or less?

Click to expand...

Master cylinder screwed in by too many turns so always fit the master cylinder precisely according to the Lockheed instructions and never be tempted to add 'one more turn'.

Seeing as you have your technical answer this wise-head recommends you move the bike and your family up here where temps. are crisp at negative numbers for upcoming X-mas time. Try driving at night or parking in the shade. What kind of drink ?, we are presently enjoying a bottle of Hardy's Butcher's Gold. Just received a new M.A.P. stainless piston rod actuating assembly for the MK 111 rear brake from the other cold place now ,U.S.A.

thanks for the replies
I am aware of the bleed hole problem and have already re-assembled the cyl once to check the body was not screwed in too far when I had trouble bleeding the brake. I am confident the assembly is correct. The brake has been operating fine for - say - 8 months or so. After it locked you could just push the bike so I rode slowly for five minutes or so and then it seemed to free up some. Perhaps some wear of pads and also relief of pressure. By the time I got home - say 15 mins - riding on the front brake, it was normal again. Hence my suspicion of it being heat related.
Also the hoses are new good quality braided S/S.
I will leave it for now and see if it happens again. I will report if it does. May save someone some grief. I am surprised no-one else has experienced this.
I will hold off on the move Toronto. We don't own any warm clothes.

shedweller said:

thanks for the replies

I will leave it for now and see if it happens again. I will report if it does. May save someone some grief. I am surprised no-one else has experienced this.
.

Click to expand...

Bear in mind that a rear brake locking on in traffic could potentially cause an accident, a nasty accident ?? - so the sooner you find the problem, or determine it has gone, the better.

Releasing the pressure at the caliper by opening the bleed valve would have at least told if if it was a caliper/piston sticking problem, or somewhere else in the system.

Brake problems are not something to be taken lightly, you rather depend on them for safe motoring. Your insurance would probably be void if they heard of this....

Had a similar problem in the Chrysler Coupe . Lock a wheel if brake stabbed . Release and count to three before reapplying .

Fully laden ( understatement :shock: ) in Arthurs pass , in the wee small hours , approaching a steep downhill lefthander ,
with low armco to the precipice of the gulch ( only 300 ft straight down ) and a patch of gravle on the approach /THREE GUESSES .
( lL front locks on Gravel . Oh Dear . ) Release Brake . . . . .

Swallow hard , count to two and a half , and apply brake GENTLY , then more firmly . Needless to say we didnt go through the armco
but wouldnt have got a third chance .

Was the bleed down bit where doesnt back of smartly , IN the Master Cylinder . Wouldnt be adverse to repair before you try it .

The ' crack the bleed nipple open ' as someeone above stated , While its NOT backing off ( when Stuck ) will eject a drop or so , if Pressureised .
i.e. indicateing it IS the Master Cylinder , NOT the piston / caliper . ( If it were the Caliper Sticking , the Brake Lever might not be as solid either,
as in itd move a little ( an iota , indeed ) Hydraulic Lock is condition if no fluid release through valve at cylinder .

If you try Arthurs Pass , as it is , fit a ejector seat & take a Parachute .Might apply in other instances also .

shedweller said:

I will leave it for now and see if it happens again.

Click to expand...

Carry a spanner for the bleed nipple. If it happens again, and momentarily opening the bleed nipple immediately cures the problem then (unless there is a blockage in the line) it can only be down to residual pressure within the system and the only place that pressure can normally vent is through the M/C bleed port. If the M/C piston was sticking and not returning fully then you should feel more free play than usual at the brake pedal.

Also, check the vent in the master cylinder reservoir cap is clear. Although fluid is isolated from air by the diaphragm, the reservoir must still be able to 'breathe'. Check that nothing is preventing the M/C operating rod from fully extending to the 'off' position when the brake is released.

You could try unscrewing the master cylinder by 1turn?

Good advice thanks Les. I didn't have a spanner on me at the time. Have one in my coat pocket now.
I couldn't really feel extra free play, which kinda goes against my theory of a locked M/C piston.
The caliper pistons are the US S/S ones from Fred'n'Ella and seemed a good "silky" fit in the bores. It has been o/cast and showers since it happened but as soon as it warms up I will go for a trot and try to duplicate the circumstances sdo I can try tthe bleeder and so determine whether it is the M/C or seized caliper pistons.
Will check the cap etc.
If all else fails I will strip the M/C again, but not looking forward to the bloody bleeding again! (sorry)

BLEEDING .

get 1/2 metre of clear plastic tube from Repco . See the Nipple is 1/4 turn open , no more . end of tube in bottle / tub .

FLUSH the system . Close nipple . DONE . ( pump cylinder quetly , Can leave 1/2 hr for bubbles to converge , before packing up )

with a ' wet ' system theres ordinarilly no bubbles . You tend to get them when all / some components are new / dry . the microscopic ones drift of and converge , overnight .

Getting NO air in the system helps . If the nipple is open more it wobbles & can suck air up the treadson the backstroke . So 1/4 to 1/3 turns it .
tecnically sling it shut with lever in . But Tube prevents back egress so t doesnt matter .

Without the tube it can suck at each backstroke and imbibe air from the bottom . This makes it real fun , if youre a masochist . :mrgreen: