I've been trying to get my 1992 D250 ready for a road trip. It's a diesel with 288,500 miles.
On random stops the brake pedal will be really sensitive. As soon as the pedal moves the truck is stopping. Most of the time there is the normal amount of free play in the pedal.
The brake booster and MC were changed within the last year.


No grinding or other sounds are heard.
The pedal effort remains the same.
Brake performance remains the same.
The truck does not pull to one side.

The only notable difference is that the brakes activate at the first movement of the pedal.

Possible culprits are:
Bad master cylinder.
Bad front brake hose.
Broken brake pad.
Stuck caliper.

This happens maybe 1 in 10 stops. Most of the time it is fine.

I pulled the truck into the shop this morning and will try to look at it today.

What do you guys think is the problem?

Booster isssue. do check the round pushrod nub clearance first (want ~.020"). post how much you have & you might increase it a bit depending on if it is very close or not

It doesn't make much sense that the booster would have a problem like that 1000 miles after installation. It happens maybe once in 10 brake applications.

I had it happen two or three times on the way to work this morning.

A1Cardone booster? Go find a good used one instead.

Also, how's the parking brake adjustment and rear shoe adjustment?

The booster works fine. When the pedal is high I still get the assist.

Parking brake and rear shoe adjustment are good.

Isn't the booster hydraulic because it's a diesel?

If so why aren't you listing the pump as a culprit?

Vacuum booster in 92.

I don't mean lack of assist, but more of lack of return.

Didn't you mention brake light problems in another thread?

And will ad, sticky booster ???

Is it a bad thing when you can't compress the caliper piston in a bench vise?

I think I found the culprit.

Bad bench vise?

Had a little trouble bleeding the brakes at work. One of the guys grabbed a power bleeder and we hooked it up. As soon as he turned it on the MC filled and started spraying fluid all over the cowl and firewall. The big flat cap wouldn't seal on my MC. Putting that aside, we bled the brakes old school.

They didn't feel 100% when I left the shop and they seemed to get a bit softer as I drove. I headed over to a quick lube place for another attempt at bleeding. Before I got there the brakes felt perfect. 100%. I pulled into the shopping center parking lot and did a few more test stops. The brakes were perfect so I headed home.

About 15 miles later as I neared home the brakes felt spongy again. A 2 foot panic stop from 45 mph had the pedal sinking very low and the truck gliding to a stop. The fronts locked up under 15 mph and I never felt the rear ABS kick in.

Convinced that there was some air in the lines I took it easy the rest of the way home.

Then, when I pulled into my driveway, I had a full firm pedal just like it's supposed to be. No squish. Quick brake reaction.

The attempt at a panic stop makes me think the MC is the culprit. There has to be something going on with that thing.

Over the 18+ years I've owned this truck it has been through several master cylinders. The latest one was a new (not reman) Bendix unit that I bought in November 2014. It has seen maybe 4000 miles of use.

ABS problem

Ding ding ding we may have a winna. It sure sounded like there was some kind of voodoo going on here as a MC can vary a bit on acting up as can sticking pads/calipers but generally not close to the extent Feets is reporting. all that is left is the newfangled ABS

The truck has rear only ABS with the pump mounted in the frame rail above the rear axle.
That would not mess with the front brakes.

New MC installed this morning. I even picked up a new Mity Vac since my old one was troublesome.
I carefully and slowly bench bled the MC. When I was happy with it, I popped off the old MC and installed the new one. After that was another round of bleeding the ABS pump and all 4 corners.

It had a soft squishy pedal.

Being a bit fed up, I took it to the corner quick lube and had them bleed the brakes.


I'm back to having a variable pedal.

Now, with the exception of the metal lines the entire front half of the brake system is new.

I guess I should go ahead and do the rears. If the front calipers were that bad I doubt the rear cylinders are much better.

I'm running out of time before my road trip.

Were you saying that the calipers were sticking earlier? Are the calipers new?

I had a similar situation happen to me with the Dart. At times, it would stop great, other times it required more effort. I was perplexed as the front end was brand new. Everything, from a new manual MC, rotors, calipers, bearings, etc. Just recently, when I sold the 7 1/4, I think I found a potential culprit... The friction material on my rear drum shoes unglued and so I practically had no rear brakes. Maybe that's helpful?

Disconnect the vacuum line to the booster and see what happens.

Samy, I will do the rears this week. They aren't much fun due to the full floating axle.

I have no reason to suspect the booster. The assist is fine. It has not varied.

I was messing with it tonight and got another squishy 2 footed panic stop. I could firm it up with a couple pumps. Giving it a touch of parking brake firmed things up significantly.

So that tells you the problem is probably in the rear brakes.

wondering if the backing plate could have some divots worn into the surfaces where the linings rest on ?