Leaky master cylinder or bad vacuum booster?

[baby g]

Got a 93 7.3 IDI non turbo with 84k miles and a strange brake problem.

Brake fluid seems to be very slowly leaking, perhaps 50-100 ml in the past 6 months - 1 year. Brakes seem to be quite soft, but they stop the truck no problem. The pedal starts off stiff before starting the rig, at which point the brakes feel squishy. There's a "sigh" -ing sound when I push on the pedal, as if air is coming from somewhere. Not sure if that's normal? Also, there seems to be a leak at the junction of the master cylinder and brake boost, see the photo.

What gives?

Thanks!

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[Cubey]

It could be both. The moisture may be a leaking master cylinder. And that can eat the rubber in a booster.

Boosters can and do go bad. I'm suspecting the one on my 89 E350 isn't great, acting like yours. I feel a little blast of air on my leg sometimes when braking, too.

I had to replace one (with seemingly the last new one in existence in 2019, to fit 84-86 vans only) on the 85 E350 right after buying it, along with the entire brake system except for the hard lines.

So, replace both the booster and the master cylinder together.

 

[Kdo58]

Replace both.

 

[u2slow]

A 'sigh' is normal AFAIK. If it continues to hiss - usually when pedal is pressed- then it is leaking (vacuum leak). My Dodge is doing this now, but the sound is just annoying. Brake assist is fine.

As far as the fluid, I would say either the seals on the bottom of the reservoir are weeping, or the MC piston seals are weeping weeping through the back. Squishy brakes would seem to point to a piston seal leak.

 

[IDIBRONCO]

Also, there seems to be a leak at the junction of the master cylinder and brake boost, see the photo.

That's probably your master cylinder leaking.

I'm suspecting the one on my 89 E350 isn't great, acting like yours. I feel a little blast of air on my leg sometimes when braking, too.

Sounds like you need to replace it.

A 'sigh' is normal AFAIK. If it continues to hiss - usually when pedal is pressed- then it is leaking (vacuum leak).

This is correct.
Back in the early 90's I was really into this one girl. She was a Chevy girl so I have no idea why I thought she was all that. She used to drive my truck and say that Ford brakes always said ***** (using a different spelling for a different meaning). The sound is normal.

 

[Old Goat]

I agree, replace them both.
If it is wet between the Booster and MC, chances the fluid is leaking
into the booster.
My Booster was Hissing, scwhooch sound when pushing in the Brake
peddle. That is a vacuum leak. I installed a Vacuum Gauge in the cab
to see what was going on. Got one good application on the peddle,
then second one, almost nothing. With the gauge you should have 22HG
of vacuum. Push the peddle with engine running, it should recover
quickly.
I would get on it before your soft peddle becomes no peddle.
Be sure to bench Bleed the MC before installing it.
Probably around $200 in parts. It`s an easy fix just a pain in the rear
when ya have to do it.
O`Really`s or NAPA will have the parts. From my experience, Boosters are
rebuilt in Mexico and MC come from China.

If you look at the bottom of all threads there is similar threads. Just noticed
3 I posted when I was working through my brake problems on my 86.
Brakes haven`t worked this good in the 10 years I have owned it.


Goat