So I've had poor brake pressure since buying this truck. I decided to just replace all the brakes thinking maybe with age some of the components were not working well.

I replaced the front disks, calipers, and pads. Bled the entire system and though braking is slightly better i still have low pressure. This weekend I'm completely rebuilding the rear drums. All brand new parts to completely replace them both. I doubt this will solve the issue but at least i will have peace of mind knowing everything is good.


The problem:

When I press the brake pedal it moves 3/4 to the floor before I have any braking power. Sometimes at low rpm like coming to a stop or sitting at a redlight the pressure lessons and I can go straight to the floor. There is always some brake pressure but pumping brings pressure up slightly.

I should not have to pump power brakes though.

This leads me to believe that the master cylinder is going out even though the thing looks damn near new but it has been on the truck since 97.

I also though maybe the brake booster doesn't have enough vac pressure and that is why I get poorer brake pressure when the engine is not under load.

I've checked the lines with no sign of leaking. The fluid levels are good and are not going down.

The rear drum pads might not be adjusted out far enough and that could explain the pedal going 3/4 to floor before feeling brakes so I will fix that over the weekend.

I'm a little confused though.

My master cylinder is from a 1973 Ford F100 with brake booster and it is a power brake unit with dual chambers. Brake lines do not have any loops but go straight down from the side of the master cylinder. There is a brake line that feeds off the line going to the smaller chamber of the master cylinder with a loop to the brake light switch.

So I'm thinking master cylinder or brake booster. Am I on the right track or is there something else I should be looking at?

 

Definitely sounds like air in the system somewhere. Could be a line leaking or air getting pulled into the master cylinder. But, I have a similar setup and actually had the brake light fail and suck in a ton of air. This problem can be hard to diagnose, since sometimes the lines will pull in air before you can see fluid leaking out.

 

Probably not it but doesn't hurt to check adjustment on rear brakes just to rule them out. I've seen it cause soft pedals on other cars.

 

Pump the brake pedal several times than step on it firmly. If the pedal sinks towards the floor, the master cylinder and/or wheel cylinder(s) are leaking.

You can usually see wheel cylinders leaks by looking at the tires, but the master cylinder leaks into the P/B booster.

 

I can pump it multiple times and then step on it firmly. All this does is give me slightly better brake pressure. It doesn't go to the floor.

No fluid on the rear wheels but I do have new brake cylinders to install this weekend and who knows what I may find once I open it all up.

The system was bled correctly by me and I flushed the entire system too. Before I disconnect the lines to rebuild the drums I will crack some bleeders and do a quick bleed check and if I find air then I will know for sure that air is getting in somewhere.

I suppose I will find out tomorrow.

 

When you swapped in the disc brakes, did you also install the Brake Pressure Differential Valve (that some peeps call a proportioning valve)?

 

My truck already had disk brakes up front from a 1973 Ford F100. It was this way when i bought the truck. I recently replaced the front brake calipers, rotors and pads to brand new kit.

The Brake Pressure Differential Valve is mounted down on the frame. It has two inlets, and three outlets. One for each front disk and one that runs back into a splitter to the rear drums.

The unit itself looks very dry.

 

IMO you are correct to suspect two possible problems: Air in the lines or misadjusted shoes. Either of these will be temporarily corrected by pumping the pedal. The latter problem often masquerades as the former.

 

Didn't get a chance to do the rear brakes today as the wife left me with the kids. Hopefully tomorrow it doesn't rain and I get get some work done. I have a new master cylinder on order just in case I need it but hopefully once the rear drums are rebuilt it will solve my issue.

 

So I started on the brakes today and the drum brake adjuster star wheel is seized. I pulled everything off but decided I would rather replace the adjuster with a new one. So I put everything back together and will do the brakes next weekend.

I did notice that my brake cylinder is bad. The rubber seals are loose and as soon as I put some pressure on it; it leaked fluid. So This might be my culprit for air in the lines.

So I was looking at the drum setup and something looks wrong. I noticed that there is no adjuster ARM nor the cable, cable guide or spring. Instead mine has just a small long spring running from the bottom of each shoe. wtf?

So I looked at my shop manual and in the manual all of those items are installed. Is it possible that my rear drums are not supposed to have auto adjustment? That option is not in my manual but I suppose it could be that this was meant to be a manual adjustment??

I could really use some input here.

Check out these pictures I took. There is nothing to adjust the adjuster!! And a picture from my 1965 Ford F100 shop manual:

 

Ok after looking through the manual further I discovered another picture of the rear drum system that matches what I have. So it looks like I jumped the gun. So I just have to buy the adjuster.

Would it make more sense to install the entire adjuster kit? (cable, spring, guide and adjuster)

 

I can't clearly see the booster in the photo to know exactly what it is but, it looks like the vacuum check valve is directly behind the MC. On a single or dual diaphragm Bumpside/Dentside booster, the check valve would be at about the 11:00 o'clock position on the face of the booster.

Did you measure the booster output rod, relative to the engagement of the MC, prior to bolting the MC to the booster? If the booster output rod is adjusted too short, this will cause a long pedal travel before any significant brake application begins to happen. If the booster output rod is adjusted too far out, it will cause the brakes to be partially applied, even when you aren't pressing down on the brake pedal. With the pedal not being applied, there should be between .005"-.010" gap between the tip of the booster output rod to the piston on the MC.

Simple procedure for adjusting the booster output rod to the MC being installed on the booster.

https://www.google.com/url?sa=t&sour...2EaVr1WtAlbHQw

When you switched to discs, did you plumb in the disc/drum brake valve from the donor truck? Pre-'67 trucks didn't come with a pressure differential brake valve and no brake warning light on the dash. If you do have the Dentside disc/drum brake valve but don't have the brake warning light/wiring installed, it is still possible to determine if the pressure differential valve spool is shifted off-center in the brake valve assembly.

Take an ohm meter and touch one lead to one of the two terminals sticking up inside the nylon pressure warning switch on the brake valve --doesn't matter which terminal. Both are linked together and are at the same electrical potential. Take the other meter lead and touch it the body of the brake valve or, a known good ground. If the meter reads open (no continuity) the pressure differential valve spool inside the assembly is centered. This is good.

If the meter reads resistance (continuity), this is bad and means the pressure differential valve spool is shifted off-center and will have to be recentered.



Aside from evacuating all the air from the brake system, the drum brakes will need to be adjusted up to get a firm, high brake pedal.

 

Self-adjusting brakes debuted in Ford Trucks in 1964. Somebody along the line removed that stuff. They don't tend to work all that great after a while, and need lubrication now and then or will stop working. I wouldn't necessarily replace the star wheel/link just because those are siezed, they should clean up real nice with a little time, a wire brush and solvent/oil, chase threads a little maybe.

Lube them up thoroughly inside with never sieze or high temp grease. The OEM links are noticeably heavier and better material than the ones included with the typical kits. Otherwise though use all the parts included in the kit, springs and everything. Check for excessive grooving/gouges on the backing plate. The shoes move and slide back and forth across these, and after a lot of miles the brakes will start hanging up on the grooves and won't retract quickly.

When you have all that stuff removed it's a good time to lube the parking brake cable too.

 

@ultraranger The valve was brought over from the 73 ford donor. I can put an ohm meter on it tomorrow. Thanks for the very detailed post.

I had the adjuster off and I was afraid I was going to damage it. It just won't budge. I can soak it and work it out I'm sure but it would be faster just to buy replacements.

I noticed online for the truck there are several adjusters for sale. Are they all basically the same thing (size) just left and right side?

Yeah I had purchased a new spring kit but the springs are not nearly as beefy as the OEM jobbies. I already decided to not use the new ones.

 

Sure it's right spring kit? I spose it was used for a number of applications. New springs "should" be better than the old ones, the high heat tends over the years to take the temper out or somesuch. That's one thing that drum brakes have to do right, is retract quick and strong. Maybe they are better than some of the crapola being produced today. Maybe find NOS, should be easy enough.