'94 Brake booster?

[ForgedCrank]

1994 Ranger 2.3, 5SP, 2WD, 90K miles

I'm getting annoyed that my brake pressure bleeds off when I'm sitting at stoplights, often ending up with the truck starting to roll if I'm not paying attention.
I don't have any fluid leaks anywhere and the levels aren't dropping so I'm sure the system is at least still a sealed loop.
At this point, I suspect it is the brake booster, but I'd like some opinions.

Tests:
- Start truck, let idle for ~30 sec to build vacuum in the booster
- Press and hold brake pedal while shutting off engine
- Result- pedal holds steady for about 15 seconds, then VERY SLOWLY starts pushing back. If I had to guess, I'd say a rate of about 1/4 inch every 15-20 seconds.

Second test
- pump up brake and hold pedal while starting engine
- result: pedal sinks 2-3 inches, a little lower than when I'm normally driving and stopping

Other notes: I don't hear any hissing or anything, but that doesn't mean much as the engine makes enough noise to drown out any light hissing, but there's nothing obvious.
Also, Where the master meets the booster, there is a rust trail going straight down the front/bottom of the booster in the shape of what was likely a brake fluid leak or spill from the distant past. It's rusted and pitted, but I don't see any holes best I can tell, it feel solid there. It is also dry, no detectable oily film at all, bone dry so that tells me it's pretty old, or at least older than when I redid the engine bay and washed and cleaned everything about a year ago.

Do you guys think I can conclude that it is the brake booster with a small leak, or would you focus more on the master cyl?

As always, appreciate any feedback you may have.

!@# Awesome picture as a reward for reading my crummy post!

 

[2011Supercab]

I'm getting annoyed that my brake pressure bleeds off when I'm sitting at stoplights, often ending up with the truck starting to roll if I'm not paying attention.

This would make me think the master cylinder is bad.

 

[RobbieD]

First issue, another thing to easily check, is that the vacuum check valve is functional and not leaking, and that its grommet is making a good seal, and isn't dry-rotted or split. If anything, just rule those out.

Second issue I'd be suspecting of the master cylinder itself.

'94 year models were good trucks.

 

[ForgedCrank]

First issue, another thing to easily check, is that the vacuum check valve is functional and not leaking, and that its grommet is making a good seal, and isn't dry-rotted or split. If anything, just rule those out.

Second issue I'd be suspecting of the master cylinder itself.

'94 year models were good trucks.

Ok this is where I started but the more I started troubleshooting, the more I got interested in the booster unit due to the leak-down with vacuum absent, so getting sidetracked by that was inevitable I suppose. So that's two votes so far. So the next question I have is... does anyone know if Partsgeek is a reputable place to buy parts? This is the one I found: Brake Master Cylinder - 1990-1994 Ford Ranger - A1 Cardone 5517-07548742
The reason I ask is that I have been unable to find anyone locally that sells the Cardone brand (which appears to be a preferred aftermarket brand based on what I'm reading). Frankly, I'd rather rebuild the OEM one so I can keep that, but I've been unable to locate kits for this thing. It is only referenced in the manual by a part number that comes up empty so I can't seem to locate a kit. If anyone has a better option, I'd love to hear it.
Meanwhile, I'll pop off the check valve tomorrow and inspect that to make sure.
Thanks guys, appreciate the feedback

 

[19Walt93]

If the pedal pushes back against your foot I'd sooner suspect the booster than a master cylinder. A master cylinder that bleeds by will make the pedal slowly sink, never push back.

 

[ForgedCrank]

If the pedal pushes back against your foot I'd sooner suspect the booster than a master cylinder. A master cylinder that bleeds by will make the pedal slowly sink, never push back.

Yea that's how I wound up looking more at the booster. That being said, I'm also having the pedal "sink" while I'm at stoplights, so I reckon I could be the lucky winner and both are bad. I figure I'll try the master cyl first since that one is easier to swap

Oh yea, and just to note... I've seen a bad booster before that popped inside or something and when you do the push-back test, that thing immediately pushed back the pedal quite a bit. This one just barely creeps and doesn't start until about 15 sec after you shut off the engine. So if it's leaking, it's a real slow leak. Thats why it has me guessing and questioning if it's really bad or not.