I think I need a new master cylinder which brand?

[lukep6470]

The story so far.

So my brakes have always dragged a bit. I always blamed the booster as I can pull the pedal up with my foot and it is ok. A new booster didn't fix the problem and so I decided to flush the system. I found the pedal didn't return by itself while bleeding the brakes.

The system is made up of the following:

Dual diaphragm booster.
P6258A Master cylinder
SSBC rear disc kit.
RRS (Commodore) Stage 2 front disc kit.

The booster push rod is adjusted to push the piston about 2-3mm and the system still gravity bleeds.

Unless someone can see a problem with this setup my question is what brand of master cylinder to go for? PBR or Protex

[boofhead]

The master cylinder you have detailed above is what I am running in both of my cars.
Now one is running the same rear disks as you are (eg My Mustang) though I have no idea if the volume is correct for the front RRS setup.

[Nuts]

My guess is there is no return spring or a weak one in the booster. What booster are you using?

[lukep6470]

The first one was an ebay special the second was from VPW.

similar to http://www.ebay.com.au/itm/BRAKE-BOOSTE ... SwPhdU6CL8

The brakes themselves work fine. The fronts lock up before the rears and you really have to mean it to lock them up so I think the volume is OK. The dragging gets worse as the car warms up.

I thought there may be some issue with the return spring in the master cylinder.

[Nuts]

The first one was an ebay special the second was from VPW.

similar to http://www.ebay.com.au/itm/BRAKE-BOOSTE ... SwPhdU6CL8

The brakes themselves work fine. The fronts lock up before the rears and you really have to mean it to lock them up so I think the volume is OK. The dragging gets worse as the car warms up.

I thought there may be some issue with the return spring in the master cylinder.

Could be either. I had a similar issue with one of those boosters. As the car heats up to normal operating temperature, your vacuum will increase and this, in my case, caused a vacuum leak to the wrong side of the diaphragm partially activating the booster. The pushrod on a stock bendix booster cannot usually be pushed in by hand. maybe try disconnecting the pushrod from the pedal and see how easy the booster can be energised especially when hot. If it's hard to push, look at the master.

[lukep6470]

I'd be pretty unlucky to get to faulty boosters in a row.

When I tried bleeding the system the pedal wouldn't return by itself until I started bleeding the front brakes and then it started to return.

The system was warm when I started so maybe the cooler brake fluid helped. Maybe there is something dragging inside the master cylinder itself in the front brake circuit.

I would've thought the spring alone in the master cylinder would push the pedal up without the engine running.

[Nuts]

Pull the master and make sure it has all the springs installed. If its ok, then it can only be the booster. I have several of your type of booster here that I have removed from 67-70 mustangs and installed factory type bendix units. They work! http://www.summitracing.com/int/parts/a ... el/mustang

Late Thought... if you disconnect the vacuum and vacate the booster does it still happen?

[lukep6470]

I actually found a very useful datasheet on an ebay ad.

So the 7" dual diaphragm booster I'm using generates 1200PSI of force.
A 7" single diaphragm booster generates 900PSI.
An 11" single booster generated 1200PSI.

So I might be overwhelming the master cylinder return spring. Anyway there is a genuine PBR master cylinder on it's way so we will see if that is the problem.

[hybrid]

I just put a no name 1" one in mine (used to be 15/16) and it seems great.

Will see how long the seals last.

[Exhausted]

Check the clearance between the booster pushrod & piston in master cylinder it could be keeping the piston from returning all the way back past the port inside the cylinder.

[lukep6470]

I've tried it with preload and no preload, it reacts the same. I did make sure the system would still gravity bleed when I set the preload.

[lukep6470]

So I've put the new MC in today. The old one was a different model. It was pipe for pipe compatible but the brake sensor was on the bottom instead of the side. Also when I got it out I found I could push the piston in with my fingers relatively easily.

I've taken the car for a quick 10min spin and so far so good. Although there is a fair amount of pedal travel and you would have to hit the pedal with both feet to lock the brakes.

[hybrid]

Different sized piston or same?
Also, did you bleed the master before installing?

[lukep6470]

No didn't bleed before installing. I gravity bled it then pumped it using one of those Supercheap self bleed kits then gravity bled it some more.

The new fluid is a different colour to the old so it was easy to tell when I got it all the way through.

The pedal is very, very firm without the engine running. Firmer that the old MC ever was.

I set the preload to about 3-4mm. It still gravity bleeds.

The pedal always did act like this. I think it is because of the big Commodore brakes on the front.

[hybrid]

No didn't bleed before installing. I gravity bled it then pumped it using one of those Supercheap self bleed kits then gravity bled it some more.

This never worked properly for me.
I did it the same way with the new MC too, and pedal had a lot of travel.

Took it back out and bench bled the master. Chalk and cheese afterwards.

I have bigger calipers on the front of mine also (PBR VT commodoor I think), which is why I went for a 1" bore instead of 15/16".
Made a nice difference.