Got a 1984 F250HD a few weeks ago. When I picked it up and drove it home, i noticed the brake pedal has to be pressed in about half way in order to get any real stopping power. However, i can feel the brake slightly apply when the pedal is gently pressed. This is the case wheter the truck is running or not. Whats even more odd is that on rare times when I drive the truck, as in its only happened two or tree times in the past couple of weeks, the brake pedal will feel normal for that drive. The master cylinder is new, not remanufactured, and is almost brand new. No leaks anywhere, and the Master is full of fluid just as it was when I picked it up. Don't want to just replace parts like crazy, would rather get a second opinion first. Thank you.

 

Sam, Welcome to FTE.
I am guessing this is a power brake truck?
When anything is new to me one of the first things I do is pull all the wheels and check the brakes.
Drums: Pull the rubber cups back on the wheel cly to make sure they are not leaking, shoe mounted in the right place, I found 2 long shoes on 1 side and long on the other side. Shoes are not down to metal.
All springs in the right place and good shape, adjusters work as should.


Disc: Pistons not sticking, calipers not sticking and can move and pads have meat on them.


I think in your case I would bleed the brakes just to make sure there is no air in the system from when the master was replaced and fluid is clean.


If you still have issues find a empty dirt lot or road, get rolling a little then lock up the brakes. You was to have the truck slide a little but not too far to run into the front wheels that should of locked.
You should see 4 wheels lock up if not look into why they did not.
Dave ----

 

Second all of this. If the MC is new then there is very few things that would cause the pedal to be "Spongey" that far into a push. As Dave noted, I'd try a good bleed first. When the MC is replaced, depending how careful someone is, sometimes there a lot of air that enters the systems.

A couple other things to look at:

Check your rear drum adjustment. If the cylinder fully retracts every time, when you push the pedal you need to take up all the extra slack. (With this usually mean shoes because with new ones there wont be as much room.)

And just check to see if you Booster is working correctly. (No leaks, the hose is supplying vacuum) Even without a booster you should have a fairly stiff pedal though. Id be more inclined to believe its air giving you a hard time.

 

The rear brake adjustment is something commonly overlooked. If you want to see if this is the problem, apply your parking brake about halfway. Does the pedal act better after this? If so, most of the problem probably is the rear shoe adjustment.

Go back and jack each rear tire off the ground, get a adjuster tool or a short screwdriver, and pry the rubber cover off the adjustment hole(if it's even there). Turn the tire by hand while you turn the adjuster with the screwdriver. Adjust them till you can't turn the tire. Then back them off till you can turn the tire, but you still have some rubbing. Do both sides like this.

 

Another likely issue is when a newly installed master cylinder hasn't been bench bled. I used to think that wouldn't matter, but have since learned otherwise.

If not bench bled, MC pushes air into the lines at the start of the system bleeding process. Shouldn't matter, other than taking slightly longer to bleed, right? But there's a valve in the system that complicates things. The exact name escapes me, but it's the valve that contains the brake pressure warning switch. The valve has a spring-loaded function that requires input pressure (from the MC) to rise above a certain value before opening and porting fluid to the wheels.

If you've got air being forced from the MC (wasn't bench bled), that air compresses in the line upstream of the valve. The valve opens somewhat, but closes early due to air's compressible nature. It's a Catch 22, because that valve needs full pressure to stay open to let all the air pass through. But any air left in the lines prevents the valve from staying open long enough.

There are two ways around this:

1) Use a pressure bleeder pot, the type that connects in place of the MC cover. This supplies a larger volume of pressurized fluid than cycling the pedal can provide with each stroke. This larger volume creates a constant pressure that fully compresses any trapped air, and thus keeps the valve open. If just cycling the pedal for bleeding, the small volume per cycle isn't enough to fully compress any trapped air.

2) Crack open the fittings where the lines from the MC enter the valve. Open and close them just like a bleeder screw as a helper applies and releases the brakes. This gets the trapped air out of those lines without having to overcome the internal resistance of the spring-loaded valve. Messy, yes, but it works.

 

I have a theory also, that bench bleeding guarantees full stroke of the master cylinder, so every last bit of air can be driven out. If you have it installed in the vehicle, you have the pedal and it's linkages, the booster and the clearance between the booster rod and the master cylinder. After all that, you may not get full stroke and bottom the master cylinder out when trying to use the pedal in the vehicle to bleed it.

 

The booster is designed to have a longer stroke than the MC. When they are designed, they always make the limiting portion of the system the MC so that you are ensured a certain amount of fluid flow.

The federal spec actually uses the full stroke of the primary and secondary circuits to determine reservoir volumes. (This is what you would see if you had a blown break line.) The primary and secondary circuit required volumes in turn are driven by the volume of the pistons and cylinders at your wheels. (At least this is how they are designed now... 35ish years ago they might not have polished up the sizing method yet?)

 

Your theory is as good as mine. Maybe it's a combination of the two. All I know is when I've replaced the MC on various vehicles, if there was no intermediate valve in the system, bench bleeding was not required. With a pressure-sensitive valve present (my F250 for example), I couldn't get a reliable firm pedal without first cracking the line feeding the valve. That tells me I should have bench bled the MC

Another factor may be the location of that pesky valve. It sits lower than the MC, so any air in the lines travels backwards towards the MC between pedal applications. This is where pressure bleeding has a big advantage, as it could keep any trapped air moving towards the bleeders.