Rear Master Cyl rebuild Mk 3

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Hi All

Amongst other issues at present I have a rebuild kit for the rear m/cyl. The manuals I have (including genuine) cover the front in detail but don't cover this topic for the rear. Can anyone describe how the guts of these things are accessed to change seals etc?

Rgds Vince
 

L.A.B.

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Info:

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


Do not disturb the nut and locknut position on the push rod unless it is absolutely necessary, and if you do need to, then measure the distance between the face of the nut and the master cylinder casting first (should be 8.7-9.3mm) and then refit the nut and locknut back in the same position.

After assembling the unit and before reinstalling and bleeding it, always check that the piston moves far enough back when at rest that it clears the small return bleed hole, do this by blowing air backwards into the barrel from the outlet side.
If air won't blow back through the port then the barrel assembly is probably screwed in too far, and the brake can either suffer from locking on or dragging problems as a result, unscrew the barrel assembly half a turn {Correction - that should read one turn} at a time and retry.
 
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LAB, once again, thanks for your help, that's a very good procedure description.

Rgds Vince
 
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L.A.B. wrote,
and the brake can either suffer from locking on or dragging problems as a result

That's very interesting. I rebuilt the rear master cylinder on mine last year with a new stainless steel master cylinder, and after a run, the rear disc is always quite warm/hot to the touch, even when I haven't been using it very much. I fitted a new lockheed caliper also and a fully floating rear disc, so I would not suspect a caliper fault such as sticking pistons as it is new. I'll try this test and see if this is the cause of the heat build up :?:

Thanks L.A.B.
 

L.A.B.

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Reggie said:
I fitted a new lockheed caliper also and a fully floating rear disc, so I would not suspect a caliper fault such as sticking pistons as it is new. I'll try this test and see if this is the cause of the heat build up

As you have the system already fitted and bled I would suggest that you remove the caliper or drop the wheel and then try to push the pads/pistons back into the caliper by levering between them with a thin piece of wood or something similar?

If the pistons can be pushed back into the caliper without too much force being applied to them then the return bleed is probably clear, and the fluid level should be seen to rise slightly in the master cylinder reservoir.

If one piston is noticeably easier to move than the other then that piston could be sticking in the caliper causing the brake pad to drag slightly causing the heat build up?

Also check that the brake pedal is returning fully, and that the brake linkage is not over-adjusted as that could also stop the master cylinder piston from returning to the fully off position?


And of course I'm sure you won't forget to pump the pistons back out again after the caliper has been refitted and before riding off!
 
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L.A.B. wrote;
try to push the pads/pistons back into the caliper by levering between them with a thin piece of wood or something similar?

I'll try that method. It'll be a lot less messy. I don't know if I would 've thought of that? :roll: I'm fairly sure that the large nut on the pushrod is not contacting the body of the unit as I have checked that before.

L.A.B. also wrote;
And of course I'm sure you won't forget to pump the pistons back out again after the caliper has been refitted and before riding off!

It wouldn't be the first time that I have forgotten.....good reminder, so once again thank you. :wink:
 

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I tried the blow-back test on a master cylinder unit I'd rebuilt recently...and it failed the test!
When I stripped the unit to find out why, I found that the check valve (the two piece rubber and plastic part that goes into the barrel first on the end of the spring) did not have the tiny bleed hole in the centre of the rubber part.
So I pulled the two valve pieces apart, and I could then see there was a small tag of rubber apparently left over from the moulding process still fixed inside it.
I pulled the tag out with some needle nose pliers, and that opened the tiny hole. After reassembling the unit, air could then be blown out through return bleed.

Two piece check valve is shown as part 3.

The diagram below is actually of the Triumph front brake master cylinder, but the Commando 850 Mk3 rear master cylinder is very similar.

Rear Master Cyl rebuild Mk 3
 
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L.A.B. Out of interest, I have pushed back the pistons in the rear caliper and there was a nice little jet of brake fluid eminating to the rear of the brake fluid resevoir. So it appears that the rear master cylinder is probably assembled and working correctly.

The pads in the rear caliper are probably nearly half worn even though the pads have only been in for about 1,500 miles. When I spin the rear wheel it seems free, so although I'm not overly concerned, there probably is something not quite right. As I have an aftermarket disc and caliper on, the caliper is mounted on an alloy plate, that is in turn bolted onto the original caliper carrier. Maybe this is slightly out of true. :?: Further investigations required when time permits.
 
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