Stiff brake pedal

Occasionally my brake pedal will get really stiff when I go to slow down. Most of the time it works fine, but I am just worried about not being able to slow down on a steep trail if it happens off-roading. I have a 91 4.0. Any one have any ideas on what might fix it? Thanks.

Sounds like you're having power booster issues. Check the vacuum line and check valve, if those appear good get a vacuum pump and make sure it's properly holding vacuum. If it's bad consider swaping to a '96 dual diaphram booster and master cylinder, more stopping power.

Thanks. I'll check it out this evening.