Need advice from car guys

[Sgt. C.J. Sabre, SASS #46770]

Okay guts, a slightly weird one here. 

I just had to put a starter on the Muddog. Strait forward enough, right? Accomplished that yesterday. Today, I fire him up to go to the gym. Brakes are hard and not very effective. Symptoms seem to indicate brake booster. Sheer coincidence? I HATE coincidence, but what else could it be? The only other thing I touched was that I put new terminals on my positive battery cable. Battery and starter are on the opposite side of the engine bay from the brake booster. 

Any thoughts?

[Rip Snorter]

I go to a couple of sites relevant to my brand of truck.  Lots of info there.  Might get you good info more quickly.  Just a thought!

[Pat Riot]

Are the brakes vacuum assisted? You may have knocked a vacuum line loose or cut it. 

[Eyesa Horg]

Odd, but you may have lost some programing when you disconnected the battery. For the very limited time I worked at a Jeep dealership, the techs would back feed power thru the power port/ cigarette lighter when they disconnected batteries.

[Eyesa Horg]

Here's from a quick Google search:

Good Luck

 

A lost Jeep TJ power brake function after a battery disconnect is likely caused by a ECU/computer glitch, a tripped fuse, or a vacuum issue rather than a physical brake failure. Common causes include a need for the ECU to relearn idle, a blown fuse during reconnection, or an accidental dislodgement of the vacuum line. 

Here is a breakdown of potential causes and solutions:

ECU Relearn Process: After a battery disconnect, the computer loses its memory. Let the Jeep idle for 5–10 minutes without hitting the road immediately to allow the Engine Control Unit (ECU) to relearn.

Blown Fuses: Check all fuses, particularly those related to the ABS or main ignition system. Incorrectly connecting the battery, even briefly, can blow fuses.

Vacuum Line Check: Ensure the brake booster vacuum line (running from the engine intake to the brake booster) was not accidentally disconnected or damaged during battery service.

Battery Connection Safety: Disconnect negative first, then positive. Reconnect positive first, then negative to avoid surging the electrical system.

Brake Switch Issues: If the brake pedal feels high or the brake lights stay on, the brake light switch may have been disturbed or needs to be reset. 

If these do not resolve the issue, a professional diagnostic is recommended to check the brake booster and master cylinder, as losing power steering/brakes is unsafe

[Eyesa Horg]

33 minutes ago, Eyesa Horg said:

Odd, but you may have lost some programing when you disconnected the battery. For the very limited time I worked at a Jeep dealership, the techs would back feed power thru the power port/ cigarette lighter when they disconnected batteries.

What the techs did was wire a 9v square battery to a male cigarette lighter type plug and just plug into the power port. It would keep the computers and radio from losing programming. I was kinda surprised the car battery didn't fry the 9v transistor radio type battery!

Edited January 23 by Eyesa Horg
Added text

[The Original Lumpy Gritz]

Look for a vacuum leak.

Engine run'n, use a spray bottle of very soapy water and spray vacuum hoses and connections.

You'll hear a change in engine sound/pitch if the soapy water gets sucked in.

[JP McLintock]

Engines gonna run pretty bad if there is a vacuum leak large enough the power assist doesn't work,  like a really high idle. 

[Sgt. C.J. Sabre, SASS #46770]

Thanks guys, got it fixed. It was as simple as it can get. The Brake Boost Check Valve blew out. A $10 universal part fixed it.