Question for you tandem brake guys....

[curly type 26]

Not something as simple as seized caliper pistons? Im sure you have checked them if not remove pads, lever pistons fully in & check free movement leave lever in position & pump brake till piston moves & traps lever. Push back again & refit pads. Are they correct pads? Good luck Curly

[collins_dan]

Chris,

I put Ray's stainless steel hoses on and they do make a difference in terms of feeling of firmness. Also, 2 are slightly shorter than the other 2. I think the shorter go on front. I think I figured out I had it wrong when one rear was fitting a lot tighter than the other. I would also recommend getting speedbleeders, makes bleeding the brakes by yourself really easy. If you decide to go that route, the size you need is SB3824L. Also, I got one for clutch slave cylinder as well.

Good Luck,

Dan

[Frank Howard]

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

Steve,

I respectfully disagree. First of all, in a tandem master, the pistons are two different sizes. As they both move the same distance, the larger piston pushes more fluid than the smaller one does. That creates more pressure.

[paddy]

I respectfully disagree. First of all, in a tandem master, the pistons are two different sizes. As they both move the same distance, the larger piston pushes more fluid than the smaller one does. That creates more pressure.

I'm going to respectfully disagree Since force = pressure x piston surface area , a smaller piston creates greater fluid pressure for a given pedal force.

At the caliper, the opposite is true: for a given fluid pressure, the force at the pad increases with a larger piston.

To think of it another way, the mechanical advantage of the system (ie force at the pad / force at pedal) is equal to the ratio of (caliper piston area / master piston area). So smaller master cylinder piston = greater force (at the expense of greater pedal travel).

Paddy

[johnsimister]

Paddy is right.

John

[Frank Howard]

Paddy, you are right. What was I thinking? I should know because when I removed my servos from my Europa, I had to go to a smaller diameter master to increase the pressure in order to make up for the lack of assist. Duh.

[Chrisrich]

Thanks everyone.
Dan, I just ordered those speed bleeders.
I should get the new lines by tomorrow -- we'll see what gives.

(PS: yes, Curley, I did rebuild the calipers, and they act pretty much as you describe. The pads are the standard street pads that RD sells.....)

Chris

[ecamiel]

Do the simple things first.
Raise the front of the car and re bleed the master cylinder. Then re bleed the brakes befor you get into everything else. It worked for me.
Eric
64 S1 with S4 dual Master

[curly type 26]

Cheers Rich i just had to check as they can give those symptons, if one piston moves & one does not. Also had it with crap pads. good luck Curly

[patrics]

Gents,
I?ll still disagree and say that I am correct that both pressures will be the same.

A step bore m/cyl is used to give more pressure in the failed state for a given pedal effort to pass regulations but in normal usage it would be the same.

So how about this

Piston diameter A = 25.4 (Primary)
Piston diameter B = 20.6 (Secondary)
Input force 1200N (300N @ 4:1 pedal ratio)

Pressure = force / Area

Primary pressure = 1200 / 506.71

= 2.368 N/mm2 or 23.68 bar

Force on secondary = 2.368*333.29
= 789.23 N

So output pressure secondary = 789.23 / 333.29
= 2.368 N/mm2 or 23.68 bar

In the failed state the secondary pressure would be:

1200 / 333.29 = 3.6 N/mm2 or 36 bar

In reality because a master cylinder doesn?t have a simultaneous cut off then the secondary could be a few bar lower.

Good enough?

Best Regards
Steve

[Chrisrich]

Well, these aeroquip lines are an improvement, but I don't think I've got this problem beat yet.
I've lost much of the spongy-ness, and there certainly is a more consistent pedal. Breaking effectiveness is better, but still feels week, i.e., still cannot lock up the wheels.
After thrashing it about and pulling into the garage, I'm measuring 105 degrees (f) at the rear and 85 at the front, both sides. This just sounds wrong to me, and surprisingly low all around, no?

[chicagojeff]

Dear all,
2 comments. First, when you say you are getting less braking force than you think you should have, did you say what pads you were using? Just a thought that you can get the car to skid or chirp with uprated pads, even with less than optimal pressure in the lines, so maybe describe more how your pedal feel is?

Second, the polite and informative disagreement on this thread just reminds me how much I love this owners group. No flaming, no bullsh**. Respectful teaching and learning. It really is part of why I own an Elan. Thank you all.

Jeff Who Must Soon Put His Car Away For the Long 8 Month Chicago Winter

[Chrisrich]

Agreed. Well said.

As to my pedal, it's hard to describe. In and of itself, it's not bad. While not the most solid pedal I've felt, there's plenty of meat to it. Probably drops 1/4 before it comes on. If I threshold brake, i.e., full-on from 50mph, I probably get 3/4 of the way down, but never to the floor, for instance. Like I said, the stopping power is improved, and is probably even usable as is, but it just doesn't have that bite. The pads are called Classic Gold. I don't know if they make more than one type of pad. I got them from Ray at RD. I bled the master (i.e, cracking the 2 output bolts while pushing down on the pedal), and I had no problem bleeding all the lines.

I'm wondering if I should try to switch the lines, to feed the front brakes from the rear of the master.

[paddy]

In reply to Steve (patrics) comment: you may well be right. I've never looked at how these tandem cylinders work, but I know it's not as simple as two independent pistons on two independent circuits just fixed to the same pushrod so they always travel the same distance. (If it worked like that, then the pedal would stop moving as soon as one of the circuits had moved as far as it will go, leaving no pressure in the other one.) My point was just about the relationship between piston size and pressure.

As for the step-bore cylinder I thought that was something different, ie a master cylinder that displaced a large fluid volume for the first part of the pedal travel (to move the pads into contact with the discs) and then a smaller piston for the later part of the travel to maximise the braking pressure.

In relation to Chris' problem have you thought of going down to your local friendly MOT centre and getting them to put the car on the brake testing rollers so you can get an accurate measurement of the brake performance front and back? At least then you'd have a clear idea of what you're dealing with. I don't have the numbers but I'm sure someone here will have the data from their last MOT to tell you what the expected efficiency should be.

Paddy

[collins_dan]

Chris, You could run new lines just in the front, as the factory set up was unnecessarily long on federal cars, going from master to right side, then splitting to the right front tire and then back across to the left front tire. If you routed it from master to splitter to left front and right front, you would reduce the length of line by about half. Dan