BMW caliper upgrade Just Not Right

[sleepdoc]

I need a bit of input on my brakes.

My 74 3.2L conversion has rebuilt stock rears, venting set at .004. Rebuilt 320i calipers in front. New pads all around. New stainless lines. New 19mm M/C. Brass brake line "Tee". I am now post multiple bleeding sessions, about two gallons of fluid.

This is a follow on from a previous post.

The original problem was the calipers were binding and the pedal was getting hard as a rock and very high when the disks got hot, leaving all four corners locked.

cle2cont gave me the pivitol idea. Since I could relieve the lockup by opening a bleed screw at any corner, he suggested the MC wasn't returning to baseline and allowing the return of fluid. I recalled that I had lengthened the MC actuating rod to give a short pedal throw, so I shortened the actuator rod and now no more binding. Ok great.

I have a firm pedal and have been working the new pads so they are biting better and better.

My current question is about pedal travel. Looking at many posts about MC and 320 calipers it appears that the bigger 320 pistons require more fluid leading to a longer pedal travel until the pads hit the caliper. This also leads to some recomendations to go to a bigger MC (19mm) to fulfill the need for more fluid movement to keep the pedal travel about stock ( and requiring a bit more force).

I have no complaint about the braking behavior, it is pretty darn good, and no complaint about the amount of force needed to actuate the system.

I am perplexed however about the pedal travel. I have the rear venting adjustment up tight, and have adjusted the actuater rod longer and longer. In order to get reliable release of the brakes after hard use( ie no binding or locking up) the actuating rod must be set so that the pedal moves about two inches before calipers start to work, then firm and normal from there.

The acuator rod is supposed to be set so that there is 2mm freeplay before it contacts the MC socket it rides against. If I set it there the pedal would move maybe three inches to firm contact. There is plenty of pedal travel left so this doesn't seem dangerous, it just isn't right.

I have the sense that the calipers are working correctly and that the MC is not behaving as expected.

I wouldn't mind a small increase in pedal travel, one of you proposed that the 320 brakes reuire 25% more fluid and the 19mm MC supplied 23% more fluid at higher foot pressure. If correct this seems close enough so that the slight increase in pedal travel should not be irritating.

Your comments and ideas are awaited with appreciation.

best Mark

[sammyg2]

Set you M/C actuating rod back to where it's supposed to be. It's not meant to be an adjustable piece to compensate for pedal travel.
Buy and install a 19mm master cylinder and you will be happy.

[Zeke]

Read it again, he HAS a 19mm M/C.

I said the 19mm M/C had 25% more piston surface area. Who said BMW calipers had 25% more fluid requirement? It's very easy to get these numbers out of context. The pistons move only a fraction of an inch, whereas the M/C piston moves a lot further. Therefore, you have to divide the M/C travel by the combined travel of all the caliper pistons before you get a true picture of what moves what.

[Bleyseng]

I have the 320i brakes and 3" of MC travel ? Are you sure you have bled the brakes right?
With my rears adjusted to about .008 of clearance I have about max 2" of MC travel.
Something seems wrong here....maybe how you have adjusted the MC arm?
Geoff

[sleepdoc]

Hi All

Zeke

Here is the quote from one of Dave Darling's posts

"Brunofski opines:
> I put the BMW 320 calipers on my 914 1.8 ... The extra volume of the BMW caliper is well compensated for by the larger displacement of the [19mm] MC.

Someone did the measurements and math for me the other day--the BMW brakes have 23% more piston area than the stock 914 ones, and the 19mm M/C has 25% more area than the stock 17mm one. Almost an identical match!

--DD"

Bleyseng,

I am as sure as I can be that the system is well bled. The
MC arm is the actuator rod I refered to and it is adjusted probably longer than it should be to reduce travel as much as possible without stopping the MC from returning to its rest position and allowing fluid to return from the lines. If I set it to spec the travel really is too long.


In the face of a satisfactorily hard pedal and good braking function could this be a symptom of partial MC failure?

best Mark

[Bleyseng]

So how much petal travel are you talking about? 3" seems like that is halfway to the floor to me, is that about right?
If that is right, I would guess something is wrong with the master cylinder like you suggest.
The difference I notice is that with the 19mm MC and stock good working brakes the petal was very near the top of the stroke. With the 320i calipers it is futher but not by much and nowhere near the halfway point.
Geoff

[CamB]

"I am perplexed however about the pedal travel. I have the rear venting adjustment up tight, and have adjusted the actuater rod longer and longer. In order to get reliable release of the brakes after hard use( ie no binding or locking up) the actuating rod must be set so that the pedal moves about two inches before calipers start to work, then firm and normal from there. "

I had exactly this symptom on my 911 - as weird as it sounds, there needed to be massive freeplay between the actuating rod and the m/c before the m/c would "release". It makes absolutely no sense but is true. It meant the first 2 inches of brake travel was required to take up the slack. Then it worked fine.

I had the m/c rebuilt and it is perfect now (if I lived in the US I would have bought a new one for $109).

Cam