Brake gurus I could use some help.

To start, I installed a CPP power booster and cylinder on my 57 F100 with stock drums at all four wheels a few years back. I've never been happy with the feel of the brakes since. Lately, I've noticed a very spongy pedal. I climbed under the truck last night and the neighbor kid helped me bleed the brakes. Now I have no brakes at all. The pedal goes all the way to the floor with no resistance other than the return spring. I suspect the master cylinder has given up the ghost, but after talking to one of my older friends, he suspects a wheel cylinder. I really don't have the cash to just blindly start replacing parts, so I would really like to know that the part I am replacing is bad. Other than visual inspection, is there a way to test a wheel cylinder? Can I disconnect one wheel at a time, plug the line and see if I can build up pressure? Or, can I disconnect three of the wheels at once, and try to build up pressure at the one wheel that's still connected? I'm thinking if I can build up pressure at any of the wheels, that would rule out the master cylinder. Am I right? If I can't build up pressure at any wheel, I would lean towards the master. I know for sure the rear wheel cylinders are new in the last couple of years, I honestly can't remember if I did the fronts too. Here's where you guys come in, is there anything I'm completely overlooking? Thanks for the help.

 

I should add, the master cylinder is a Corvette master. When I do a little research, it looks like it's for a 67-74, but those all have disc brakes and my truck has drums. Would that explain the soft pedal all these years? If so, can anyone recommend a good drum/drum master cylinder to use in place of the Corvette one? Thanks again, John

 

If it never felt good? Is there a chance that there is no free-play before the brake push rod touches the master cylinder?? If there is no play there, it will not bleed correctly.

Do you have a proportioning/safety valve for the system? A safety valve might now be stuck to one side which makes bleeding impossible.

Blocking fittings: Old time well stocked auto parts stores do have block-off fittings for standard sized fittings. Master cylinders sometimes do have odd size fittings, so you might not find anything available to block one side off at a time.

 

First off, were you losing fluid all along? What you describe goes along with a leak somewhere, if fluid can get out, air can get in. It could be anything from a bad master cylinder cover gasket to a wheel cylinder, line or hose. Look for a trail running down the firewall from the master. Those are hard to see sometimes.

Then go the usual route, look for leaky wheel cylinders, bad lines and hoses. Loose connections can seep over time and not be that obvious.

For a master, why not go with something close to what you have? I'd use a 1967 and up F-100 master for drum brakes.

Bob

 

I'm not really sure what you mean, so this could be the problem.

No prop valve, no safety valve. Just a master cylinder running to wheel cylinders.

Thanks

 

Pressure is equal to force multiplied by area. force remaining constant, the way to increase pressure is by using a smaller diameter piston for the master. Check the diameter of the piston for the stock master and for the corvette master--my guess is that the corvette mast is a larger diameter (thus the need to compensate with a booster for additional force to achieve a similar line pressure). That kind of abuse may have blown seal out in the master. Have someone listen next to each wheel to hear the wheel cylinder engage. If one of them is out check for wet spots on the backing plates. Good luck~

 

I haven't noticed any fluid leaking anywhere, though I haven't really gotten into it to inspect anything yet.

Thanks Bob, if the spacing of the mounting holes on the flange is the same as the one I'm running now, I'll go that way.

 

Are you using the stock manual or boosted pedal, or an assembly supplied with the CPP master and booster? What is the pedal ratio? (should be about 3:1 with the booster) Did you check to be sure the pedal bottoms out the master before it hits the floor? Now would be a good time to check this-open up a front and rear bleeder and fully stroke the pedal.

The Corvette disc/disc master works great with drum brakes-it dosen't know the difference, or cares. If there is a chamber size difference, the larger should go to the fronts. Disc masters have no residuals, but you should have them with drum brakes, so add a 10 lb residual to each axle, and locate them as close to the master as possible.

I would pull the drums and check for obvious leakage or damage, and stuck wheel cylinders. Pull the dust boots away at the bottoms of the cylinders-any fluid present means they are leaking. Make sure the brakes are all adjusted properly.

When bleeding, purge any vacuum in the booster (engine off), make sure the reservoir stays topped off, and fully stroke the pedal slowly, having someone close the bleeder(s) before the pedal is released. A pressure bleeder, if possible, would make this job much easier amd faster.

Bob

 

Like Bob396 says; use a master system designed for 4 wheel drums. The Chevy Pickup from 67 to 70 uses 4 drums. There is a chance that GM used the same master mounting bolt pattern as your vette cylinder.

You really should use a safety valve when using a dual master. Without one, you are basically running a single circuit brake system if one line pops. The safety valve has a piston inside; it blocks off the blown circuit during a failure.

The safety valve for the 67-70 GM pickup looks like a piece of 3/4" square stock about 5" long with 4 fitting holes on it. I have no clue where to get a new one. Try ECI Brake? The owner is on hamb. ECI GUY or something like that.

One question: Is the master cylinder under the floor? If so, you supposedy need check valves inline to prevent the fluid from gravity flowing back to the master.

 

V8Bob, I was writing while you posted. Is he better off with the chevy 67-70 cylinder for 4 wheel drums at this point? I would think that the truck cylinder would be pretty cheap compared to retrofitting residual valves?

 

I did forget, as mentioned, to check the push rod clearance-should be about .060" (1/16th") with the pedal FULLY retracted.

There is no such thing as a safety valve.(??) The pressure differential SWITCH shuttles when there is a pressure loss or differential of about 400 psi, and simply turns a brake warning light on in the dash, but does not/can not block/divert/prevent fluid flow. These "diff" switches stand alone, or are part of combination valves, including proportioning and/or metering, nether required or needed on a drum/drum system.
Bob

 

It's the stock pedal assembly for the Ford. I've always thought the pedal travels to about one inch off the floor before the lever stops in the cylinder, but now that you mention it, it may be bottoming out on the assembly bracket. I'll definitely look at this. I've put residual valves at the top of the list of things to get.

No the master is on the firewall. So, residual valves AND safety valves?

 

Push rod clearance? Not to sound stupid, but clearance between what and what? The push rod just bolts to the pedal on my truck. So, there's no safety valve??? and I DON'T need a residual valve???

 

As I tried to say before; there needs to be a gap in between the end of the rod that goes into the master cyl. That gap or free play makes sure that the piston in the master can return completely. If the piston is not returning all the way, it can't bleed correctly, plus the brakes may hold a bit of pressure to the wheel cyls.




I am still up in the air on the GM safety valve statements; My 72 Chevy disc/drum Master and Proportioning valve installed in my 66 pickup definately does block one side. My proporting valve did get stuck in one direction during the build, and the rears were fully blocked while bleeding and driving. The brake pedal was ok and I did a quick roadtest and the rear drums were cold. I had to struggle to get it to bleed with the shuttle stuck like that.

I have a 68 pickup GM service manual someplace...I guess I should look up the 4 wheel drum valve to see exactly how it functions. Geez, I was pretty sure it blocks off one side???

 

Oh, got it. Thanks for all the help guys. I'll check it out tonight, in the meantime, please keep the tips coming.

 

I've done a few dual master conversions along the way. On drum-drum conversions, I've used the 4-way splitter block and nothing else. Usually the donor car gives that up if I'm parts shopping in the boneyards.

I did this on an old Mustang, Fairlane and my '65 Belair and they all stopped well.

Bob

 

This information may be already be stated in this thread, but I am going to put it all in one place for you since I think it is the most likely source of your problem. You need to check all your linkage clearances.

1. The rod that attaches to the brake pedal. Check to make sure it is tight. Did the Ford pedal use a 3/8" pin with matching hole and the CPP part a 5/16" pin? Doesn't sound like much, but it can be a problem.
2. That same rod goes into the booster. It should be threaded so you can adjust it. Disconnect the return spring from the brake pedal, pull the pedal up to the stop that limits how far the spring can take the pedal up, and see how much you can push the pedal in with basically finger pressure. Adjust it to the spec in the manual or the one given here.
3. The rod that goes from the master into the booster. When mixing and matching parts, this can be a really sneaky tripping point. Pull off the master and look into the booster. You will see a cup that will receive the rod on the master. Make sure that they will mate properly. When you attach the master cylinder, make sure that the rod seats into that cup long before the flange is on the booster. If it doesn't, you will hit the pedal and the booster will move without moving the master.

If you still have problems after you do this, then you have a tougher road ahead. I have swapped brakes on a number of cars and the connection points I outlined are the biggest sources of problems in my experience.

 

I'm still a little foggy on the 4-way splitter. I assume at least one of the 4 ports on the slpitter is an "in" from the master cylinder, but what about the other three? Isn't the point of a dual chamber master cylinder to seperate the fronts from the backs?

 

Johnboy,
If your system is matched it is probably OK, but the push rod clearance refered to is between the push rod, either out of the booster or from the pedal, to the primary master piston. The booster push rod just connects to the pedal, as you stated.

The stock pedal ratio in your truck is about 6:1 if it is a manual system, way too much for a power assist. You need to change it about 3:1.

Go back and re-read-I stated you need residual valves, NOT a prop valve.

F&J,
The dual master is the "safety" valve. That's it's purpose-to isolate the primary and secondary brake systems. Can the "diff" switch fail and possibly cause a blockage, yes, but something I've never seen or had happen. That's one of many reasons I always recommend NOT to use anything but an adjustable prop valve in disc/disc/ & disc/drum systems, along with the necessary residual valves (10 lb to all drums, 2 lb only to discs when the master is below the floor) and NO metering or "diff" switches, including "combo" valves, in custom brake systems.

Bob

 

Thanks, you've made it crystal clear. I'll look into it tonight.

 

Did you run the master cylinder dry while you were bleeding it? If so then you probably need to start over by "bench bleeding" the m/cyl. It can be done on the car. If you let the m/cyl go dry then you can let air into the m/cyl that was not there before. Start there to confirm that the m/cyl has no internal air.

Since it was on the road, if you lost fluid it would show up somewhere... cracked brake line, leaky w/cyl (will usually show up on the tire), the fluid has to go somewhere... unless the seals in the m/cyl have given out. In that case the piston seals lets the fluid pass by internally and no fluid is lost. If you can not find any wet spots in the system then it has to be the m/cyl.

 

Again, not to sound stupid, I really don't know. When you say 3:1, what do you mean? I understand that it's a ratio, but what units are you referring to? 3 what : 1 what? Do you mean pedal length to distance traveled? Do I need to either shorten the pedal, swap it out with one from a power boosted vehicle, or do away with the power booster? Also, I apologize if I'm missing it, but I don't see where I misquoted you about the prop valve or residual valve.

 

It didn't run dry, but it was low. It didn't look like it was lower than the port position though. Could be something else to look into, thanks. There is absolutely no evidence of brake fluid on the tires. I did check that while I was bleeding the system last night.

 

Johnboy,
The "4-way splitter" is a pressure differential switch. These are stand-alone or part of combination vavles. There are two inlets, one from each port of the master, and two outlets, one to each axle or system. (Most front-drive vehicles have "cross-split" systems, with one front and one rear on one circuit, where most rear drive have the conventional front/rear or verticle split hydraulic systems) These switches were common until fluid level -warning switches, built into the masters, made them obsolete.
Bob

 

Seriously, thank you. I really appreciate the time you guys are taking to help me out.

 

I just went home for lunch, and checked a couple of things out while I was there. There is just enough travel in the pedal before I can feel resistance to let the electric brake light switch close completely. With the pedal fully depressed, I can still slide my fingers under the arm of the pedal all the way up to the bushing the pedal mounts on. This set-up was specifically made for a 57-60 Ford truck, so I would hope that this has been all engineered out. I think this rules out anything related to pedal travel. The brakes worked at one point, I just wasn't happy with how far down the pedal went when you really needed to stop. I'll check the wheel cylinders tonight. BTTT for the afternoon crowd.

 

Many years ago I suffered a similar problem and couldn't find any fluid leaking. Long story, traced the problem to a master cylinder leaking into the brake booster, the reason for no visible leakage. Actually took the vacuum brake booster apart and drained the fluid. Good luck

 

Before you go off the deep end adjust up all the brakes. We are used to front disc brakes with no front adjustment. 4W drums require more adjustment maintenance than we are used to. Get everything up to par on your equipment before you get too far afield. A little adjustment front and rear can make a great deal of a difference. More so than a standard disc/drum system.

JMO.

 

Thanks for the tip, that was number one on my to do list.

 

Well, I just came in from adjusting the front brakes, they were both really out. I bled the brakes again afterward and I got brakes now, but they're still no better than they've always been. When driving, I get no braking action until I'm well past halfway down, then they come on strong. I have to go to class tonight, so I'm done tinkering for the night. Tomorrow, I'll check and adjust the rears if needed.