1939 master cylinder

[Woodie1]

I want to replace the master cylinder on our 1939 Ford. I'm thinking if I remove the brake lines at the rear fitting & remove the wires to the brake switch, I should be able to remove the 3 mounting bolts for the cylinder, & the controls at the front of the cylinder can be left alone. Am I seeing this right or am I missing something?

[Kube]

Originally Posted by Woodie1

I want to replace the master cylinder on our 1939 Ford. I'm thinking if I remove the brake lines at the rear fitting & remove the wires to the brake switch, I should be able to remove the 3 mounting bolts for the cylinder, & the controls at the front of the cylinder can be left alone. Am I seeing this right or am I missing something?

You should, with some patience, be able to R&R the cylinder with the lines connected to the brass fitting. Simply remove the bolt with the switch.

[Woodie1]

I hadn't thought of that Kube. That would save me a little time.

[Flathead Fever]

Bleed the master cylinder before you hook up the lines.

[rich b]

Anneal the copper washers; whether new or old being reused.

It would be a good idea to make sure the pushrod adjustment is free while the master is off; just in case it is needed with the new master.

[Woodie1]

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I took the new master cylinder off. I could not bleed the brakes. Most of what I read said it wasn't necessary to bleed the single port master. I'm trying to bench bleed it with no luck. I keep getting weeping at the rear of the cylinder & air bubbles keep forming at the intake port. I put on a new brass fitting at the rear of the master along with 2 new washers. I have the brake switch mounted. What am I missing? I'm about ready to get a rebuild kit for the old master & try bleeding that one.

[Kube]

Originally Posted by Woodie1

I took the new master cylinder off. I could not bleed the brakes. Most of what I read said it wasn't necessary to bleed the single port master. I'm trying to bench bleed it with no luck. I keep getting weeping at the rear of the cylinder & air bubbles keep forming at the intake port. I put on a new brass fitting at the rear of the master along with 2 new washers. I have the brake switch mounted. What am I missing? I'm about ready to get a rebuild kit for the old master & try bleeding that one.

It's possible the master is defective. It happens. There is no need to bench bleed a single cylinder master.
The weeping? That's probably your issue. If it's weeping fluid, it is sucking air. You need to get those washers to seal completely.

[Woodie1]

I have read on here that the bench bleeding wasn't needed. I was getting desperate as I can't get a good brake pedal by bleeding the wheel cylinders. I've tightened the fittings as much as I dare. It was mentioned to anneal the washers.

[Terry,OH]

The master cylinder piston has a seal around it at the end toward the operating rod, if that seal leaks it willl allow air to enter.

[Woodie1]

Does the angle the car sits at make a difference in the bleed out of the air in the wheel cylinders? I read where 1 person had lowered his front axle for the bleeding process.

[petehoovie]

Originally Posted by Terry,OH

The master cylinder piston has a seal around it at the end toward the operating rod, if that seal leaks it willl allow air to enter.

[Woodie1]

I got tired of bench bleeding the master. I put it on the car. I had the 2 brake line holes plugged. The brake pedal was hard & didn't go down. I hooked up the front brake line & the pedal was still hard. Hooked up the rear brake line & now the pedal was low. I had the brake shoes tight against the drums so I wasn't loosing movement there. Now I wonder if the new master is bad. We bled all 4 wheels 4 times around & no improvement.

[mhsprecher]

What order did you bleed the brakes?

[Woodie1]

Passenger rear, driver rear, passenger front, driver front was the sequence.

[Kube]

As Terry and I (posts 7 & 9) have suggested, a defective master is a possibility. Me? At this point I'd obtain a replacement.
NAPA sells NEW (not rebuilt) cylinders that I have had nothing but good fortune with. About $55.
The listing, when you go there, is under 1948 Ford.

[Woodie1]

I will get a replacement master. It just surprises me that I had good pressure with half of the brake lines connected & bad with the other half connected also.

[FlatheadTed]

Could be a displacement issue ,to big wheel cylinders or to small M/C -1,1/8 ,one pipe hooked up the piston can fill the front ect .

unlikely ,but is this a Duel out let ? Could be You had one out let blocked so the fluid was backed up against that rendering the other one not working ,you then hooked up the other pipe allowing fluid flow into it result soft padal ,

[Terry,OH]

If the fronts are on and the pedal is good I would look at the rear brake system. Maybe block the flex hose and then see if there is still pedal.

[Woodie1]

I misspoke myself. The rears gave good pedal & the fronts went soft & low.

[Woodie1]

I got a new master from Napa. It is better but I still get air no mater how may times we bleed the system. I think we bled at least 8 times all around. I also lengthened the master push rod. I get braking after the pedal goes down about 3".

[rich b]

Originally Posted by Woodie1

I also lengthened the master push rod.

Can you explain that?

Did you adjust the pushrod to provide discernible clearance between the pushrod and master cylinder piston with the pedal at rest pulled back by it's spring?

[Woodie1]

When i move the pedal by hand under the car, I am unable to determine at what point the rod contacts the piston. Either way, I am unable to expel all the air bubbles. I don't see any bubbles after a third bleed at each wheel but when I come around for another try, bubbles show up again. just can't find where they are getting in.

[Kube]

Originally Posted by Woodie1

When i move the pedal by hand under the car, I am unable to determine at what point the rod contacts the piston. Either way, I am unable to expel all the air bubbles. I don't see any bubbles after a third bleed at each wheel but when I come around for another try, bubbles show up again. just can't find where they are getting in.

You shouldn't be having this much trouble. You knew that

How exactly (the process) are you bleeding?

[Woodie1]

The first new mc I put on, I did not bench bleed to start with. Later I did bench bleed it. The new mc I put on last I did not bench bleed. I have a tire tires removed with the car blocked up. I put the wrench on the bleed screw & have my wife pump the brake pedal 3 times slowly & hold down on the 3rd pump. I open the bleeder & keep it open until I don't see bubbles through the hose or into the bottle. Close the bleeder & she will release the pedaal. When the pedal is up, she pumps 3 times slowly again & holds down. I open the screw & watch for bubbles, & then thighten screw when no more bubbles show. We do it one more time. We start with the passenger rear wheel, driver rear wheel, Passenger front & then driver front. After each wheel is bled, I add more fluid to the master. I put the cap back on the master each time but only until it meets the cap seal. I don't tighten it. I am using a bottle with a check valve & a tube submerged in fluid. I have even wrapped the bleeder screws with teflon tape. The screws feel quite loose when they are loosened. I'm open for any ideas. I've read of putting a 1' thick board under the pedal so it can't go to the floor. I haven't tried that yet.

[Kube]

Originally Posted by Woodie1

The first new mc I put on, I did not bench bleed to start with. Later I did bench bleed it. The new mc I put on last I did not bench bleed. I have a tire tires removed with the car blocked up. I put the wrench on the bleed screw & have my wife pump the brake pedal 3 times slowly & hold down on the 3rd pump. I open the bleeder & keep it open until I don't see bubbles through the hose or into the bottle. Close the bleeder & she will release the pedaal. When the pedal is up, she pumps 3 times slowly again & holds down. I open the screw & watch for bubbles, & then thighten screw when no more bubbles show. We do it one more time. We start with the passenger rear wheel, driver rear wheel, Passenger front & then driver front. After each wheel is bled, I add more fluid to the master. I put the cap back on the master each time but only until it meets the cap seal. I don't tighten it. I am using a bottle with a check valve & a tube submerged in fluid. I have even wrapped the bleeder screws with teflon tape. The screws feel quite loose when they are loosened. I'm open for any ideas. I've read of putting a 1' thick board under the pedal so it can't go to the floor. I haven't tried that yet.

Thank you for this detail. I am confident there are others, beside myself, that would really like to help you solve this issue.
It sounds as if you are bleeding the system properly.

I'd asked for the details as I tend to go at things methodically.

Next question of you don't mind...
When you are done bleeding and everything is tightened up, can you pump the brake pedal up until it is firm?
If yes, hold foot pressure on the pedal for perhaps a full minute. The pedal should stay where you started from. Does it?

[Woodie1]

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When we are done bleeding now, the pedal is fairly firm. I can put pressure on the pedal for a minute & it appears to stay . It doesn't go to the floor. I just don't know how long it will act like this.

[Kube]

Originally Posted by Woodie1

When we are done bleeding now, the pedal is fairly firm. I can put pressure on the pedal for a minute & it appears to stay . It doesn't go to the floor. I just don't know how long it will act like this.

If the pedal holds it height for a minute or so and stays firm, there is most likely no bleeding issue.

How far does the pedal go down before braking begins?

[Woodie1]

I would say the pedal goes down a couple inches before braking starts.

[Kube]

Originally Posted by Woodie1

I would say the pedal goes down a couple inches before braking starts.

Two inches isn't terrible. I prefer a bit less than that.
My suggestion? Adjust the rod at the master a little bit at a time. That will bring your pedal travel up.

I'm really starting to think you do not have a bleeding issue.

[1942deluxe]

Mike, if you adjust the master cylinder rod out it should reduce the pedal travel or the opposite? Some of this thread may apply to what I'm finding on my 46 so I'm following.

[rich b]

As far as I know there is only one proper pushrod adjustment; when the pedal is held against the floor by the return spring, there should be 1/16" between the end of the pushrod and the master cylinder piston.

If adjusted more than this there will be wasted motion before the brakes are actuated and if adjusted less than this it could stop the piston from returning fully, possibly to the extent of not releasing the brakes.

Of course you need to be sure the pedal is free and not binding, the return spring is in place, and the master cylinder bore is not corroded hindering the piston from returning fully.

[1942deluxe]

Thanks, Rich. That's helpful.

[Kube]

Originally Posted by 1942deluxe

Mike, if you adjust the master cylinder rod out it should reduce the pedal travel or the opposite? Some of this thread may apply to what I'm finding on my 46 so I'm following.

Making the rod longer reduces pedal travel. In other words, the actual braking begins earlier.
When I restore a car, I like to find the "sweet spot" when adjusting that rod. The sweet spot, as I define it is to obtain the least pedal travel prior to actuating the braking while not restricting the fluid from returning (in)to the master.
It takes me a few attempts but is worth the effort. Well, to me it is...

[Terry,OH]

Ford Spec. for the pedal adjustment is 1/4 to 1/2" free play of the brake pedal. There should be no need add some length to the actuating rod since it has an adjustment on the pedal end, it should be 3.62" long. Also the brake pedal return spring has to be attached to bring the pedal all the way back to its proper resting place, since the free play is measured from pedal rest. Below you can see the adjustment and the fixed length of the operating rod, plus the original 39 brake and clutch pedal return spring.