Master cylinder woes

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Nillevang

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24
Allright guys. 1974 swb barn find (well actually it was in the field) have been sitting stationary for 20 years or so. I'm now doing it up, and is trying to sort the brakes at the moment. BUT the bloody thing over brakes hugely on the rear, I can't even block the front wheels on a muddy grass field. My car being a 1974 has the sls 10" drums all round, but the master cylinder I ordered is a NRC6096 which is for post 1980 models. But can that alone account for the poor braking?

By the way it's dual line servo assisted.

Thanks
 
Nillevang said:
Allright guys. 1974 swb barn find (well actually it was in the field) have been sitting stationary for 20 years or so. I'm now doing it up, and is trying to sort the brakes at the moment. BUT the bloody thing over brakes hugely on the rear, I can't even block the front wheels on a muddy grass field. My car being a 1974 has the sls 10" drums all round, but the master cylinder I ordered is a NRC6096 which is for post 1980 models. But can that alone account for the poor braking?

By the way it's dual line servo assisted.

Thanks
Click to expand...

Is there a brake compensator in the system? If so, I suspect it needs resetting.
 
It's properly bled with pressure fed through the reservoir. And the front drums are adjusted to sticktion and then back a nodge. But maybe I've bodged the leading/trailing shoe thingy... Do you know of proper guide for checking that - front and rear?
 
That's the wrong master cylinder. Its designed for the 11" TLS fronts brakes, I think you need 569671.

Not sure if that would account for all your problems, I'm pretty sure all series tandem master cylinders have the front and rear bore sizes matched but it certainly wont help.

You should have larger slave on the front on a 10" setup. I found that even with the correct master cylinder the rear seemed to lock prematurely
 
Nillevang said:
It's properly bled with pressure fed through the reservoir. And the front drums are adjusted to sticktion and then back a nodge. But maybe I've bodged the leading/trailing shoe thingy... Do you know of proper guide for checking that - front and rear?
Click to expand...

No, I just do it, been doing it for 40 years, it seems obvious.

I would guess there is still air in the front lines, these systems are quite tricky to bleed.

Most people get better results with gravity feed or traditional pumping with an assistant than pressure bleed. Some find jacking up the front end so the master cylinder is level helps, I have never had to resort to that.
 
Are the springs fitted correctly on your front brake shoes? If 10" spring goes from shoe post to post by adjuster not other shoe.
Also some shoes have a reputation for the adjuster post to fall off [Mintex the latest culprit]
 
Citizen Kane said:
That's the wrong master cylinder. Its designed for the 11" TLS fronts brakes, I think you need 569671.

Not sure if that would account for all your problems, I'm pretty sure all series tandem master cylinders have the front and rear bore sizes matched but it certainly wont help.

You should have larger slave on the front on a 10" setup. I found that even with the correct master cylinder the rear seemed to lock prematurely
Click to expand...

I do think you're right. I'll order the right one straight away...
 
Turboman said:
No, I just do it, been doing it for 40 years, it seems obvious.

I would guess there is still air in the front lines, these systems are quite tricky to bleed.

Most people get better results with gravity feed or traditional pumping with an assistant than pressure bleed. Some find jacking up the front end so the master cylinder is level helps, I have never had to resort to that.
Click to expand...

But is it possible to mount the brake shoes wrong? It's really thoroughly bled; both front and rear.
 
Blackburn said:
Are the springs fitted correctly on your front brake shoes? If 10" spring goes from shoe post to post by adjuster not other shoe.
Also some shoes have a reputation for the adjuster post to fall off [Mintex the latest culprit]
Click to expand...

I'll ensure that first thing tomorrow ;-)
 
Nillevang said:
I do think you're right. I'll order the right one straight away...
Click to expand...

Probably best, but, as he says^^^^^, not sure if it will make any difference to your issue.
The back wheels will always tend to lock up first, because there is very little weight on the back wheels under braking on an unladen vehicle.
But there should be plenty on the front brakes as well.
My guess is that you have a bleeding/adjustment issue as well.
 
Another mistake some make is putting the slave cylinder on upside down so the bleed nipple is facing down. Can you post some pics of your brake shoes etc.

Col
 
88 brakes.JPG
If you have done a full axle rebuild it is also possible to fit the brake back plates on the wrong side.The back plate shown is drivers side note the adjuster is at the front and should be on all wheels. The shoes themselves are handed and only the leading shoe has the adjuster pin on the rear for the top spring to attach too.
The other thing to check rear slave cylinders are smaller than fronts.
 
The which pipe goes where on the master cylinder (ie front and back) is a bit of a nightmare as they change and some of the sources are wrong. Check and check again. I see you tried it both ways (not a bad plan). Looking at what you have checked already my bet is wrong master cylinder but there are so many other things that can be got wrong yet look right.
 
My understanding is that Series tandem master cylinders have a single bore size who's diameter is adjusted to match either the single or twin leading shoe setup so it shouldn't mind which port goes where or which size master cylinder you use when it comes to brake balance. You have the larger master cylinder so this will require more effort but with less travel to achieve the braking effort. The 10" SLS system bleads up pretty easy, as other have said, check the way you have setup the shoes and the slaves are the correct way round.
 
Citizen Kane said:
I found that even with the correct master cylinder the rear seemed to lock prematurely
Click to expand...
On my previous SWB (with 10" brakes all round) I also found this - I used to put the best tyres on the back axle, which helped a bit.
I read somewhere that the SWB front brakes were upgraded to TLS 11" in 1980 because of a legal requirement that the front brakes must lock first.
I agree with Citizen Kane's post - it shouldn't make that much difference if the m/c is the wrong size, or if the rear brakes are connected to the front or rear port on the m/c, but it is important that the slave cylinders are all correct, both front to back and side to side
 
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