Dual Stage Master Cylinder Issues

[Matthew Devereux]

This is turning out to be another upgrade I regret. I installed the Zims dual stage master cylinder kit last year. Ever since then the brakes don't engage until the pedal is nearly half way to the floor and the car pulls to right a bit under heavy breaking. The pedal is hard and the car stops fine. I've bled the system and adjusted the brakes numerous times. The pedal rod is adjusted for 1mm play and the parking brake cable is backed off. The hoses seem to be in good shape and not too old. Today I took off the front left drum to watch the cylinders activate as the pedal is pushed. They don't move until the pedal is halfway to the floor as expected. I'll probably do the same on the other wheels but I'm expecting the same. Any ideas on what to check next?

[Richard Shilling]

It sounds like you are compressing air in the system with the first pedal movement. Let's start off with model and year. Disc or drum brakes?

[Matthew Devereux]

Hi Richard. Its in my sig....'58 356A which is drum of course. The pedal is solid so I'm assuming no air. Its been bled multiple times with my power bleeder. Adding insult to injury I just broke one of the bleeder screws.

[Richard Shilling]

This is from my memory of 45 years ago so everyone can tell me how wrong I am. I don't have a 356A to look at to verify. My memory says that some of the wheel cylinders have the bleed screw at the bottom, rather than the top. So bleeding with a pressure bleeder lets the fluid pass under a bubble of air at the top of the wheel cylinder. The trick is "surge bleeding". Open the bleed screw, quick stab at the brake pedal, quickly close the bleed screw. The idea is that the surge of fluid drives out some of the air. Repeat two or three times.

[Mervyn Hyde]

If the A is similar to the B, there are brake bleeders top AND bottom on each side at the front. I do like my Promotive Bleeder, but I always finish off with an 'assistant' in the car working the brake pedal (as described above) to ensure any residual air is removed. I have been left with a dodgy pedal in the past if I only use the Promotive or gravity bleeding.

[Vic Skirmants]

This is from my memory of 45 years ago so everyone can tell me how wrong I am. I don't have a 356A to look at to verify. My memory says that some of the wheel cylinders have the bleed screw at the bottom, rather than the top. So bleeding with a pressure bleeder lets the fluid pass under a bubble of air at the top of the wheel cylinder. The trick is "surge bleeding". Open the bleed screw, quick stab at the brake pedal, quickly close the bleed screw. The idea is that the surge of fluid drives out some of the air. Repeat two or three times.

NO!!! No 356 brake cylinder EVER had a bleeder screw on the bottom!

[Mervyn Hyde]

Mine does Vic. Two single-action wheel cylinders on the front, and on the rear wheels a double-action cylinder. There is a bleed valve on the bottom and the top of the front (lower in the image). Is this wrong?

[John Brooks]

You have a wheel cylinder reversed. Look at the other side for another bottom bleeder and swap them so they are all on top. You can't squeeze out a bubble from the bottom of a column

[Mervyn Hyde]

I have never had them off John. The original shoes and linings were still on there. Some images may help.mine are like these, top and bottom, about 30 degrees from the horizontal.

[Martin Benade]

As Vic said, no bleeder at the bottom of the CYLINDER. However the fronts do have a bleeder (and a cylinder) at the bottom of the WHEEL.

[Mervyn Hyde]

Thanks Martin. I thought I was going crazy, but Vic was responding to Richards post . It would seem that there are two bleeders on the front drums, opposite to each other (about 30deg off vertical). Matthew it might be a good idea to try the pedal pumping process with an ‘assistant’.

[Martin Benade]

Yes, I have bled very many cars over the years but still find a 356 with a dual MC harder than average to really get all the air out. As an experiment you might try adjusting the shoes a bit too tight to see if that helps. (but don't drive it that way) If your drums are a little out of round it becomes harder to adjust for a high pedal but no dragging.

[Steve Turino]

Mathew
I am about to install this same dual master (Zim's) in my 61 B and would love to know the resolution to this soft pedal problem. I also use this same Promotive bleeder but will have a friend (experienced mechanic) helping me on the dual master install. My car also pulls to the right under heavy braking, which I suppose is better than to the left.
Also wondering what brake fluid to use. I have read the brake fluid thread and there seems to be controversy regarding DOT 4 vs DOT 5. I am leaning toward using DOT 4 non silicone. ATE 200, Is this suitable? I use it in my Boxster.

ATE 200.png (11.59 KiB) Viewed 1607 times

[Matthew Devereux]

Its not a soft pedal issue but a low pedal issue. I found 4 spare adjusters in some cylinders and back plates. 3 broke getting them out. I put the front left drum on and will pull the rear drum with the broken adjuster. My current theory is that there is something wrong with the master cylinder on the rear brake side. It would be nice if it was only air or a bad slave cylinder but we will see.

[Matthew Devereux]

Its back together. I had to get a $10 brake spring tool to remove the big spring. I haven't found anything wrong. The pedal is still solid but it drops about 3 inches before anything happens. Its like the pedal rod needs to be longer.