Disk brake issue on T6 conversion

[John Montgomery]

I am new to 356 ownership and hoping to get some advice.
Bought a T6 which has C disk brakes. The pedal feel has never had sufficient "bite" so I have replaced the master cylinder (which apparently came out of a Microbus!) put in new fluid, checked the brake lines (seem to be new) and put in new pads. Pedal pressure is good, but I have to stand on the pedal to get them to work (the wheels will eventually lock, but it feels like the pedal may bend or break).
As I havent driven a good C or CS for comparison, I dont know how they should feel - but I am sure not like this.
I have driven this set up for about 200 miles and recently pulled out the pads and checked for even wear with a micrometer. They are of equal thickness to within a half a mm all around the pad.
Any advice?
many thanks
John

[Larry Coreth]

If my info is correct the VW Bus master cylinder (>1970) is 22mm in diameter vs. the std. 356 M/C which is 19mm. Now such an increase in diameter translates to a 25% decrease in mechanical advantage. Now in terms of your foot force on the pedal this means a 25% increase required to achieve the same pressure at the caliper piston as the 19mm dia. M/C.
Substituting a larger M/C for “better brakes” is a classic old wives’ tale based on a lack of technical (hydraulic) understanding. What fools most people is the increase in pedal firmness with a larger dia. M/C but the decrease in braking force is not as readily noticed. So bigger is NOT always better !


Thus with a VW Bus M/C this would account for your lack of braking effort !

[John Montgomery]

Larry,
thanks for your reply and apologies for the delay in responding - somehow I missed your post - I think thats the answer - now I need to figure out how to swap back!
John

[John Montgomery]

Larry, my original post was unclear - The Microbus master cylinder was replaced with with a dual circuit 911 m/c - my understanding is that the 911 m/c is also 19mm, the same as the 356 m/c.
If so, that may not be the problem - any other ideas?
thanks
John

[Kevin Wills]

John, the disc brakes are notoriously hard to bleed. Some tricks that helped me.

- rap on the caliper with a rubber mallet between bleeds to dislodge stuck air
- use a set of brake pads with no linings to bleed with (don't know why this helps, but it does)
- work one caliper for a while so you are not chasing the air around the lines
- I have also had good luck with a vacuum bleeder (not a mityvac, but the kind you hook up to an air compressor)

HTH,
-Kevin-

[Larry Coreth]

John,
Couple suggestions, verify all the pistons in the calipers are moving and not stuck ! Do this by removing one brake pad at a time, inserting a piece of wood of about half the pad's thickness (as insurance that the piston does not come out). Have someone press the brake pedal to the floor and observe the piston come out. If it does not move or hesitates, you have found a rebuild candidate. If it moves smoothly, you're OK, and retract the piston. Move on to the next piston until all 8 are done.
This is a good time to double check if any air is still in the lines or calipers.
Finally find another356 C to compare your brakes to or someone with lots of 356C seat time to test your brakes. I say this if you are used to modern boosted (power) brakes as none-boosted brakes such as 356’s may seem to require much more pedal effort than normal. Being an old fart I grew up with none-boosted brakes and personally find today's brakes way too sensitive for precise modulation but that is what ABS is for, I hear!!
OH yes early 911/912 M/C's are 19mm dia.

[John Montgomery]

Thanks Larry and Kevin - good suggestions both - worth trying and I will report back.
Re the pedal pressure Larry - I also drive an old 911 without boost so I know (and like) the feel of unassisted brakes (I prefer unassisted everything actually!) - the best way I can describe the 356 is that there is no bite - the pedal takes high, but one needs to REALLY put the boot in before meaningful retardation. Feels almost like there is grease on the pads (there isn't). BTW, if it helps, before I upgraded the M/C the same problem was evident.
thanks again
John

[Vic Skirmants]

HMM...high pedal; have you tried Larry's advice to see if any pistons are stuck?
Also, when pushing the piston back, open the bleeder screw. This will make it easier, and you can see if any more air bubbles come out. It's also not a bad way to freshen up the fluid; pushing the old stuff out.

[John Montgomery]

Thanks Vic, checked for stuck pistons this weekend - all good - its looks like the calipers may have been rebuilt recently as they are freshly painted and look pretty new.
Will try a pressure bleed and see what happens.
John

[Ernesto Cabrera]

John, since you have a conversion to disc, check the diameter of the caliper pistons. Standard is 48mm in the front calipers and 35mm in the rear. If the pistons are smaller than these you will have to put a lot more force on the pedal to effect the same pressure on the disc. Should they be the correct seize or larger (pistons) then you most likely have a problem with the friction material try using a different more aggressive pad. You did not mention a brake releasing problem so I assume that your fluid flow thru your brake lines are fine.

Best

Ernie Cabrera

[Vic Skirmants]

Actually, a smaller piston in the calipers will require LESS force at the pedal.

[Ernesto Cabrera]

Smaller pistons have less mechanical advantage at the master cylinder so, there is less travel at the pedal but more pressure is required: P1/V1=P2/V2 if temperature is a constant.

Ernie

[Vic Skirmants]

Looks like I'll have to dig out my old mechanical engineering books and slide rule.

[Ashley Page]

I agree with Ernie and Larry. Check the size of the master cylinder - it should be 19mm as that is the way it came from the factory. Also agree that if you don't know the history of the system then a set of new pads may give you some answers. (Mintex red box are very cheep and IMHO also very good - I think Zims has them)

If you change calipers to ones with smaller diameter pistons (keeping the same master cylinder) then more pedal effort is required to achieve the same clamping force you had before the change.

If you change the master cylinder to one with a smaller bore (keeping the same calipers) less pedal effort is required for the same clamping force at the expense of a longer pedal, because of less fluid displacement per increment of travel with the smaller piston in the new master cylinder.

The inverse is also true.

The master is sized based on fluid displacement required by the calipers and to give a reasonable pedal feel and travel. If you upset the system with different parts you are going to change the feel.

If you like a firmer pedal, but not at the expense of requiring more pedal effort, then you can get that to some effect by only changing the oem style flexible brake lines to dot approved steel braded teflon lines (size -3).

A firmer pedal is easier to modulate than a pedal with more travel.

[Jacques Lefriant]

Hi John
i would change out the lines "seem new" may not be the case. Take what Ashley said into account.
j