Question for you tandem brake guys....

[Chrisrich]

We have two outputs on our masters, an 'A' (closer to the pushrod) and 'B' (further).
Can you tell me which side should be servicing the front, and which the back?

On my '70 S4 Elan, the 'A' output services the back, and the 'B' is on the front.

I suspect this might be wrong, as while I have the master out of the car and on my bench, the 'A' output has a LOT more push than the 'B' output.

I've been trying to diagnose sub-par braking for quite a while now....

Thanks- Chris

[patrics]

Hi Chris,

Closer to push rod or primary normally feeds the front brakes

Regards
Steve

[collins_dan]

Chris, Mine is the same as your's, front pressurizes front, back pressurizes back, but mine has no issues. What do you mean by sub-par? Feeling of firmness, travel, distance to stop...
Dan '70 S4 SE (Federal)

[Chrisrich]

Thanks guys.
Dan, I got my S4 a few years ago, and after 5 or 6 months of mechanical resto, I always felt like the brakes were not as they should be. For instance, from 40 or 50 mph, I cannot lock the brakes, or get the tires to chirp or bite. It's like I have ABS, only without the good part of ABS. I'm determined not to make them right, as I've had one or two near-misses.

I've taken the rotors to be turned, but while I'm waiting for them I pulled the piping to the master to see what kind of pressure I had at the outputs. The rear seemed strong, but the front was almost undetectable. The rear also has a banjo fitting, and the front has a straight 3/16 compression fitting (i.e., the front is smaller than the rear, but maybe this is just a clearance accommodation). This master was rebuilt by White Post, and in fact, they just sent it back to me saying nothing is wrong with it (lower output at the front).

The rear also has a larger reservoir, I believe? It certainly makes sense that the greater pressure output would be closest to the pushrod, and that you'd want to deliver that to the FRONT, no? Having said all that, the piping in my car looks to be somewhat 'original' (i.e., piping very orderly), so I'm not quite clear on what's going on.

[Chrisrich]

Dan, check out this page, about 3/4 of the way down:

elan-f15/brake-problem-t18323.html

Seems to show the rear of the master hitting the front brakes!

[collins_dan]

Maybe I'm not recalling correctly. It's easy enough to trace the lines. That image is a Triumph dual circuit system, but you would think they would be the same. Dan

[Chrisrich]

I'd be curious, Dan, as I'm about to redo the lines in my car -- no small feat.

[collins_dan]

Mine are original also, so there are miles of unnecessary line as the master cylinder was originally intended to be on the right side. When it was placed on the left side to be a federal model, they just ran the line from the master cylinder over to the right side, then back to the left side to the left front tire.

I'll check as soon as I can, but I've been traveling all week, so the honey-do list might preclude me from doing it until tomorrow AM.

I have to say I'm fairly certain that when I was bleeding the brakes, I started with the back, which meant topping up the back reservoir, then did the front so topped up the front reservoir, not realizing this was inconsistent with the image I had posted.

Dan

[Galwaylotus]

I've been investigating this for my S3 and I have a drawing of a tandem master cylinder that I plan to use. It's from AP Racing and clearly shows the port closest to the pushrod as primary (front brakes). The pressure at each port should be the same as a tandem cylinder uses the pressure in the primary chamber to push the secondary seal.

Fred Puhn's Brake Handbook has the following description of tandem master cylinders: "Each section of a tandem master cylinder works as a single cylinder. Each has fluid-inlet and compensating ports. The piston closest to the brake pedal is the primary piston and the other is the secondary piston. Usually the primary operates the front brakes and the secondary the rear brakes.
As the driver operates the brake pedal, the primary piston moves forward, closing the primary compensating port. Fluid is trapped between the primary and secondary piston. The trapped fluid cannot compress, so it moves the secondary piston forward to seal the secondary compensating port. With both compensating ports closed, trapped fluid builds pressure in front of the secondary piston, and between primary and secondary pistons. Except for the primary-piston return spring, there is no direct mechanical link moving the secondary piston - only pressure from trapped fluid.

If the master cylinder has a constant bore, therefore, the pressure should be the same at each port.

[Frank Howard]

Anybody know what the part numbers are for the tandem master and the rebuild kit? My car came with the tandem master and even though it is an SE, it did not come with a servo. Are there different tandem masters with different bore sizes for servo and non-servo equipped cars? Any help would be appreciated.

[gjz30075]

Frank, according to the Robinson/Ross book, the master has a part number of 64068928. This is for S4 and Sprint, LHD, USA only.

There are three different master cylinder part numbers for the various S4/Sprints. The others are:
64068324 for RHD and 64068325 for S3, S4 and Sprint, LHD.

Greg Z

[collins_dan]

Chris, It is as I thought, the opposite of the Triumph illustration. Front feeds front and back feeds back, and sounds like it is equal pressure to both, so that is likely not the cause of your brake weakness. If you do opt to switch it around and it makes a difference that would certainly be interesting to know. Dan

[Chrisrich]

Well, that was a very helpful posting from GalwayLotus.
It seems like you are correct, that the current setup should be correct. I don't really want to replumb the lines, as they have different fittings on them (to clear the clutch master).
I'm going to try one more pass at everything else before I circle back around to changing those pipes. Hopefully truing those rotors will have helped. Unfortunately it's been raining here for a few days now (we share the weather with the US Open, which is suffering worse problems right now....)

Thanks-

Chris

[patrics]

Hi Chris,

Even though it would be normal to pipe primary to the front brakes it shouldn?t make much difference to the performance of the system.

If there was two different size master cylinder pistons then the pressure would still be the same in each split.

Have you checked or refurbished the flexible hoses?

Regards
Steve

[Chrisrich]

Steve, that's my next (last??) step, I think: the hoses.
I'll call Ray at RD and see if he can send me a set.
I test-rode the car again, and brakes are not much better, if at all.
The first pump is very mushy, and it only gets slightly firmer after that. But even then, I only feel like I've got 60% of the braking I should have....
Don't really want to drive the car until it's sorted.