Hey all,

I've been running into some issues with my 74 F250 recently. I have always had some slight braking issues with this truck (brakes were always a little bit spongy), so I finally got around to changing the master cylinder.

Now all of a sudden, I am having some weird issues. I went to bleed the brakes, and I was getting fantastic pressure at the front brakes, but little pressure at the back. Furthermore, when I press the brake pedal down, it seems like there is absolutely no pressure until about halfway down the pedal-throw, and then it is hard as a rock. After messing around for a few days, I decided to try a new MC. I exchanged mine on warranty and put it in last night. While I didn't have someone with me to check pressure at the back wheels when bleeding (used the one-man method with some tubing and a bottle), I am still getting that same feeling in the pedal.

In case it matters, this truck is equipped with front disc rear drum.

What could the issue be? Brake proportioning valve? A collapsed line? Just a lot of air still? Really scratching my head at this and would love to get this unit back on the road.

Any input would be greatly appreciated!

 

When the master cylinder is replaced the push rod length needs to be checked to make sure it's within specs.

 

Disc brake pressure is higher than drum. Do you get proper rear braking?

 

When driving, it definitely feels like there is no braking in the rear

 

All kinds of possibilities here, starting with a failure at the master, crud in the master, crud in the lines, distribution blocks etc.

As to the bleeding, I would try to get someone to help you so you can make sure you're not sucking air back in.

 

Update...

Adjusted the pushrod - a lot of the free travel is gone. Feels like normal.

However, the brake warning light on the dash stayed on.

Took the master cylinder off again, took a very tiny screw driver and reset the shuttle valve inside the brake proportioning valve (or whatever it is actually called). Brake warning light turns off. As soon as I hit the brakes again - brake light returns, brakes are very spongy, until they get pumped up.

Could this just mean air in the rear lines?

Hoping to never have to bleed brakes in my lifetime again!

 

In all of this I did not see it posted that the master was bench bleed before install.
If it was spongy before all this then it could of had air in the system before starting.

Also as old as the truck is I would look into replacing all the rubber brake lines.
They may look good on the outside but be bad on the inside and either not let fluid to the wheels or back out to the master.

I have also found that the only way I could get the air out of my system was with a pressure bleeder.
Then again my system was open for a year as I rebuilt the truck, cab off frame.
Dave ----

 

Mind if I ask what pressure bleeder you used? I’m thinking that the OP as well as myself could benefit from finding a good one.

 

Hey Dave,

I should have mentioned that I did do a bench bleed. I also tried bleeding with another person, bleeding with a coke bottle, gravity bleeding. My friend even has a rig that he build out of an old shop vac that hooks up to each bleeder and pulls fluid through - tried that as well.

Maybe I will try a pressure bleeder. That should confirm no air and eliminate that possibility.

Makes sense about the rubber hoses too. I’ll order new ones just as a precaution!

 

"Motive" brake bleeder and "Mighty vac" vacuum pump have been the main stream shop tools for a long time. The mighty vac comes either with a plastic housing or metal. The metal one is easier to maintain in my opinion but for the budget minded the plastic housing vacuum pump does the job well just not as long. They both have a lot of accessories to make them applicable to many set ups. The one area that using a power bleeder makes vulnerable is the rubber line part of your brake system. If the rubber lines are deteriorating in the inside sufficiently it will create a vacuum lock there where the brakes will remain somewhat engaged where the shoes or pads will drag so best to start with new rubber lines if you've gotten to the part of brake system maintenance where you're replacing the master cylinder.