Hot Rods 1" master cylinder with disc/disc. Help needed urgently.

I've had to run a Speedway disc conversion on the 9" rear end of my rod because I simply cannot find the parts to put stock drums on it. They may be common as dirt in the USA but where I live they all seem to have been weighed in for scrap prices.

So, I have discs front and back, and a 1" Speedway Master Cylinder.

But the problem is I'm getting no back brakes. Everything has been bench bled, and re-bled and checked and double checked.

here's the MC I'm using, but I've since noticed on the Speedway site that they say: Our new dual feed master cylinder will work with most disc/drum combinations.



I'm reading on the inter-web that there will not be enough fluid flow for the rear discs.

Have I goofed up and got the wrong part?

I have a trans problem too but I need brakes to at least be able to test things properly. I want to get this on the road this week.

 

just from the looks of it I'd say that it is a disc/drum Mcyl. 2 different size reservoirs....

I've always used parts that matched the parts that I am using. I want a Mcyl that came from a factory equipped 4W disc brake system. I personally don't care for aftermarket brake part other than the brackets to mount the factory parts that can be found in a good parts store. This is probably because there were no aftermarket "kit brakes" when I started out.

 

Paul i just checked the catalog and i'm pretty sure thats a disc/drum cylinder, part no 910 31420

 

It should still work. Yes it is a disc drum MC. Plumb the larger reservoir to the front. Depress the pedal and hold it down and check the fluid level in the smaller reservoir and you will find it is still more than 3/4 full. You may want to upgrade to a vette MC as it has large front and rear reservoirs. I ran a drum drum MC when I upgraded to disc on the front for a summer. Worked fine and the reservoir did not drain dry with the brakes where applied.
That doesn't help with the no brakes at the rear.

 

Paul just a thought but do you have residual valves fitted ???

 

I had a similar problem before. No rear brakes. I had too much air in the new lines and hardware. Needed to open each bleeder, have the pedal depressed, close bleeder, pedal up, repeat. I got lucky, hopefully that is all it is.

 

size of the reservior shouldn't matter too much after the system if bleed & filled! Just keep your eye on it.
JimV

 

Which rear calipers are you using? Early Gm disc?

 

just a thought did you bleed the master cylinder make sure that the mc is working right

 

Another fact not disclosed; Where is the master cylinder mounted? and what is the vehicle??

 

Quote . . . . "I'm reading on the inter-web that there will not be enough fluid flow for the rear discs".

Most disc brakes take LESS fluid flow to function than drums do.

Caliper pistons not move much . . . . , slave cylinders for drums do.

May want to worry more about the mechanical advantage built into the pedal.

Take care, K

 

I did have them on both front and back, plus a proportional regulator to the rear, but I have now plumbed the rear direct to see if they were causing the problem. It made no difference.

I've bled and bled and stripped and bled again, even removed the MC and made fittings to bench bleed it as the cheap plastic hardware that came with it seemed to leak air in. Still, no brakes at the rear. The pistons move a little bit, but like I saw on Google, they say if the system was originally designed for drums it may not move enough fluid with discs.

Just about everything I use has to be manufactured or modified, or imported from the USA. I guess I should have thought a bit more about it and looked for a disc/disc MC when I ordered my stuff from Speedway, but it never occurred to me.

I guess if I blocked off the brake pipe on one side at the rear the other should operate the brake correctly, just to see if my theory is correct about not flowing enough fluid volume?

Anybody know of a MC they can recommend? Maybe the Corvette one, like Flathead suggests. It'll take time though, and I wanna go for a drive this week. And the trans still needs fixing.....

 

Then why don't they sell only one M/CL if it doesn't matter?

You always run this risk when you mix and match unrelated parts. IMHO. One size does not fit all.

 

Do NOT overlook the fact that the M/C may be defective. We're seeing more and more of this happening...

Also, make sure you have a tiny bit of play in the pushrod when the pedal is released. If not, you may be covering up the compensating port before the rear cylinder is full.

 

From memory the brake slave cylinder is a 1/3 the diameter of the disc brake piston though.

Leverage is about 1:5 ratio from memory, I can check tomorrow. It is 10pm here now.

Master cylinder is a couple of inches below the disc brake calipers, under the drivers seats.

 

When bench bleeding it it appears to flow fluid in one bleed pipe for the first half of the stroke, the rear circuit, then the front circuit for the second half of the stroke. (or maybe the other way round, it has been a looong day)

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake1.htm

Looking at the lowest diagram, the one with the 'leak' on, that would match what mine is doing, pedal drops 50% then goes fairly hard, but still no back brakes. Maybe the front brakes need more bleeding? But it would also indicate maybe I don't have enough leverage for the rears?

Damn, I've never had a problem bleeding or putting together brake systems before.

I do have some slack/play in the push-rod, about 1/16".

 

When it is bled (and working properly!), fluid should flow equally at all times from both ports. You just found your problem.

 

But when bench bleeding there is no resistance to fluid leaving the front portion, so nothing to pressurise the rear piston, almost like I had a leak on the front, as shown in the diagram I mention above. 'How a master cylinder with a leak works'.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake1.htm

Maybe I should do more work on the front calipers. Maybe the problem starts there.

 

just a thought, but is the master getting full stroke, and is the 1" daim. sufficient to match the needs of the disc reservoirs?

 

Which rear calipers are you using?
If early Gm, you gave to "rachet" them out or they will never work right.
Everytime you push the pedal the pedal will go to the ground, then you can pump them for a good pedal. The calipers will take one pump to get the pads just to the calipers.
Especially if you are using a 1" which is too small bore bc it does not produce enough volume of flow as a larger bore will produce.

Disc brakes take MORE volume to operate than wheel cylinders.
You only need a 3/4 bore master for wheel cylinders.

A 1" bore Will work for disc disc, they just don't work as well as they should.
A disc drum master is made internally to give more volume to the front disc brakes and less volume to the rear.
That is why disc disc master is better for a disc disc set up on the truck.

A 1" will give more stopping power than a larger bore as needed,
But the pedal will always seem a little lower tha it should be.

Do your brakes bleed fine? Just have to pump them once to have a good pedal?
Again, which rear calipers?

 

checked your bleed nipples are at the TOP of the caliper, ie you have the LH capier on the LH side etc? common error with conversions.

 

Also. Are you sure the calipers are not on upside down / right and left side switched/ or they gave you 2 rights or 2 lefts?
The bleeder screws have to point UP.

 

Pro, you beat me to it!
What he said.

 

Also what customrod said.
More specifically, what is the measurement that the rod moves if you take the
Master off and see how far it pushes master?
More or less than 1.5",
How much exactly?

 

The calipers are the correct way up, bleed nipple at the top.

I did ratchet the parking brakes in to take up all the slack. There is less than a millimetre between the pistons and the pads now.

The master cylinder pushes in maybe 1 1/4" if i remember right. I used a Philips screw driver to push with when bench bleeding it.





Now I look at the pics, I have the push-rod connected to the 3rd hole from the end, which should give about 5:1 ratio.

 

When bleeding do you have the rotors clamped secure, like with the tire on? If the rotor 'hat' is sitting on the studs and loose you won't get the brakes to work right.
The master cylinder has a 'lead/lag' built in where one set will receive pressure before the other - lets you stop nice and straight. It shouldn't be very much but seem like a lot if you check it with the pedal travel.
The disc brakes need only a teaspoon of fluid operate, it is dynamic pressure that is telegraphed not fluid.
Do you have a high point in your rear brake line, where it becomes rubber and drops down to the rearend? That is where i would look for air. When building the race cars the rear tires are so huge and that area is 'busy' anyway i put a tee at the highest point and add a remote bleeder so's it can be bled from the trunk.

 

Yes, rotors are clamped by the wheels, everything is solid. And yes, there is a high point, maybe 10" where the brake pipe goes up along the top rail then down to the axle. But the pipe is only 3/16 so fluid speed/viscosity should push the bubbles down and out, theoretically...

I was hoping to drive it a little down our quiet road, bed the brakes in a bit, as that always seems to make them feel better. Surprising how a lot of travel in the pedal when in the garage turns out to feel very small when you are actually braking when driving. However, trans problems mean I can't drive it yet.

 

Are your brake lines seperate front to rear or tied together? seperate them if they are together on the same line.Then check the piston movement on the pedal,you may be low on piston movement in the m/c,due to ratio of pedal to piston movement. be sure at the bottom of the pedal you are at the bottom of the m/c piston stroke. Best I can do,I hope it helps you. Eddie

 

You may be right, but I've never found one that acted that way when bench bleeding. Remember, you are looking at "differential" pressure on a closed system (with a leak). When bleeding, neither circuit has pressure, so the only resistance comes from the return spring behind the inner piston. Why would the center spring necessarily compress before that one?

Sorry, don't mean to bog your thread down with details. Like I said, every properly working master cylinder that I have bench bled has had equal fluid flowing from both ports.

Your mileage may vary...

 

Since you have the m/c below the calipers, U need a 2# residual valve in the line to retain brake pressure at the wheels. Speedway sells them too.This fixed my roadster brake problem. Eddie