Master cylinder question.

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DaveEFI
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Master cylinder question.

Post by DaveEFI »

Got an SD1 with dual circuit brakes. Two outputs on the master cylinder. Models with solid discs have a front/rear split. Injection cars (mine) with vented discs, two pipes to each front caliper and extra pipework - but the same master cylinder. The output ports are a different size - the front 12mm, the rear 10mm (same as all other male unions on the system)

I bought a kit of new pipes from Automec. Only one has a 12mm male union. It is a long pipe which crosses to the LH - part of the second circuit to the front calipers. For other reasons, I looked at the old original pipes. And the only one with a 12mm union is a short one that runs from the master cylinder to the front/rear balance valve just below the master cylinder. So with the new kit of pipes, the two ports on the master cylinder would be swapped.

Must admit to not understand how these tandem master cylinders work. If swapping would make and difference in practice. But I'm not going to risk it, so have made up new pipes so I can do it as original. Other thing I don't understand is both pipes to the master cylinder are the standard 3/16" despite one port being larger.

I can't remember a job that has given me so much grief. Couldn't find anything online showing the brake pipes for the late models. The BL parts fiche that has been around for some time only shows the early pipework.


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stevieturbo
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Re: Master cylinder question.

Post by stevieturbo »

Is the master a single bore diameter ? ( most are )

If so, shouldnt really matter too much what you do although it will affect any split should there be a leak
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Re: Master cylinder question.

Post by ChrisJC »

I would think that the different size ports are to make the assembly process idiotproof!

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Re: Master cylinder question.

Post by DaveEFI »

ChrisJC wrote:
Sat Jul 24, 2021 4:04 pm
I would think that the different size ports are to make the assembly process idiotproof!

Chris.
Because the pipes stick up quite a way with no master cylinder, it wouldn't be difficult to confuse them. Which begs the question - are they different for a reason?
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Re: Master cylinder question.

Post by ChrisJC »

I suppose that depends on the internals of the master cylinder. If the bore is consistent within both stages, then it doesn't matter which output is which. If one has a larger bore than the other, then maybe it does??

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Re: Master cylinder question.

Post by SuperV8 »

DaveEFI wrote:
Sat Jul 24, 2021 8:54 am
Got an SD1 with dual circuit brakes. Two outputs on the master cylinder. Models with solid discs have a front/rear split. Injection cars (mine) with vented discs, two pipes to each front caliper and extra pipework - but the same master cylinder. The output ports are a different size - the front 12mm, the rear 10mm (same as all other male unions on the system)

I bought a kit of new pipes from Automec. Only one has a 12mm male union. It is a long pipe which crosses to the LH - part of the second circuit to the front calipers. For other reasons, I looked at the old original pipes. And the only one with a 12mm union is a short one that runs from the master cylinder to the front/rear balance valve just below the master cylinder. So with the new kit of pipes, the two ports on the master cylinder would be swapped.

Must admit to not understand how these tandem master cylinders work. If swapping would make and difference in practice. But I'm not going to risk it, so have made up new pipes so I can do it as original. Other thing I don't understand is both pipes to the master cylinder are the standard 3/16" despite one port being larger.

I can't remember a job that has given me so much grief. Couldn't find anything online showing the brake pipes for the late models. The BL parts fiche that has been around for some time only shows the early pipework.
Can you share a part number from the master cylinder?
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Re: Master cylinder question.

Post by SuperV8 »

Is this your master cylinder?
https://brakeparts.co.uk/shop/Rover/SD1 ... es/BMC4328

The smaller port is P = which means primary piston
The larger port is S = which means secondary piston

The two sizes is just so they don't get mixed up on assembly.

I think its all to do with the reservoir and having more fluid capacity for the primary piston, so if you spring a leak braking will be preserved for longer to the primary piston.
This master cylinder has the same size primary and secondary pistons so it won't effect braking function swapping the pipes over - but I would still keep it as is.
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Re: Master cylinder question.

Post by DaveEFI »

Thanks to all for the explanations. I decided to make up new pipes with the correct unions. Now it's all assembled, I'm glad I did, as them crossing over would have been very obvious and look odd (to me) - and they may even have rubbed on one another.

Had I noticed the unions were on the wrong pipes to start with, I'd not have needed to make up new pipes as they both had to be shortened anyway. So trivial to swap the unions when doing so.
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Re: Master cylinder question.

Post by DaveEFI »

Anyone give any tips on brake bleeding? Odd, as I've done it lots and lots of times on a variety of cars, including this one. I normally use an EeziBleed, as it makes it a one man job. Also have a vacuum bleeder - but the EB is my choice as it's faster. Started out with 20 psi in the tyre.

The SD1 only has one rear bleed nipple. Did that first and got some 'pedal' Moved on to the front LH then RH and pedal went to the floor. Thinking there must be a leak somewhere, cleaned every single union with brake cleaner and kitchen paper towel. No sign of a leak. Did it all again, making sure I was following the BL manual sequence. And again. Have flushed through 3 litres of new fluid, but still air trapped somewhere.

Only thing is the car is parked up on a slight slope - front high. But I'm sure I've bled the brakes OK with it facing this way before.

As I said, bleeding the rear gave some 'pedal'. But leave it a few minutes and it goes to the floor. Is it possible to have a leak with draws in air, but doesn't leak fluid?

I started out with a new (NOS) master cylinder. Thinking it might be faulty, changed back to the original, which wasn't leaking - just decided to fit a new one along with the all new pipework.
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Re: Master cylinder question.

Post by stevieturbo »

Pressure and vac bleeders are great...but sometimes the basic pumping the pedal and holding it, then release a nipple in turn, just makes that little bit of a difference ( albeit it requires 2 people )

That said, often I just use a large syringe and piece of clear tubing. Ideally also either put some compound on the nipple threads, or ptfe tape first though as if you suck with the syringe, you can see air bubbles in the tube from the threads and think it's air in the system.

But the syringe gives you easy visual access, and access to both draw fluid through the system, as well as push it back in to try and bleed both directions.
Plus it's dirt cheap and simple.

The car being on a slight incline wouldn't make a difference really, wouldn't be worried about that
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Re: Master cylinder question.

Post by SuperV8 »

I would suggest its either trapped air or leaking internally - between the master cylinder piston seal if you can't see an external leak at any joints/caliper pistons or wheel cylinders?

If trapped air you should be able to pump the pedal a few times to get a hard pedal - and it should stay hard with your foot pressed on the pedal (ooh er :shock: )

If leaking internally the pedal would still sink.

I don't like the 'pump and hold' two person method - as there is a chance of the master cylinder picking up some 'crap' down the bore where is never usually goes - and then you'll have an 'internal' leak past the master cylinder piston seal. Not such an issue with a new cylinder but still. If you do use this method don't let the pedal sink to the floor.
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Re: Master cylinder question.

Post by DaveEFI »

Since I was doing major work, I got a new brake cylinder. Then changed back to the old one. Same problem with both. What I replaced was all the steel brake pipes. With a copper kit from Automec. I removed the old, and bent up the new to match as best I could, then fitted them. The only one I didn't do this with was the long one to the rear. Where it goes into the balance valve, it has a curve where it changes from up to down, rather than two bends to make the top run horizontal. And the top of that curve is higher than the original pipe, due to it being longer. Is a curve more likely to trap air? I'm really clutching at straws here. Due to never having had the problem in 60 years of playing with cars. :D
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Re: Master cylinder question.

Post by SuperV8 »

DaveEFI wrote:
Wed Aug 04, 2021 8:38 am
Since I was doing major work, I got a new brake cylinder. Then changed back to the old one. Same problem with both. What I replaced was all the steel brake pipes. With a copper kit from Automec. I removed the old, and bent up the new to match as best I could, then fitted them. The only one I didn't do this with was the long one to the rear. Where it goes into the balance valve, it has a curve where it changes from up to down, rather than two bends to make the top run horizontal. And the top of that curve is higher than the original pipe, due to it being longer. Is a curve more likely to trap air? I'm really clutching at straws here. Due to never having had the problem in 60 years of playing with cars. :D
If it is trapped air - you should be able to pump the brake pedal and get a firm pedal, and hold your foot on the pedal and it should not drop to the floor.

If it drops to the floor then its a leak.
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Re: Master cylinder question.

Post by stevieturbo »

The pipes are too small for air locks to be an issue.

However....control valves etc can cause issues. What is the nature of the balance valve ?
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Re: Master cylinder question.

Post by DaveEFI »

stevieturbo wrote:
Wed Aug 04, 2021 9:14 pm
The pipes are too small for air locks to be an issue.

However....control valves etc can cause issues. What is the nature of the balance valve ?
My mechanic neighbour was available today. And after trying conventional bleeding, he came to the conclusion the balance valve is faulty. After we swapped master cylinders once more. The flow from the rear bleed nipple is tiny compared to the fronts. And we agreed the only thing that could cause this is a faulty balance valve.

There is a problem there. Rimmer list two - one for all SD1 except the Vitesse. Vitesse one NLA. No mention of my EFI which has the same brakes as the Vitesse.
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