I just changed one in a 97 blazer Their well known to go out the diafram leaks
advance auto sells them $100.
The relay is on the left side of the power brake booster/.
Against the driver's side firewall It's the giant circular unit behind your master cylinder!
power booster behind master cylinder sounds like they may be the problem
check the relays above the brake booster check the relays above the brake booster relay above brake booster
Just a suggestion, check your trailer lights harness for corrosion. They were taking the dash lights down in my chevy.
DOT 3 Brake Fluid
If a 1991 Chevrolet S10 Blazer warning brake light always stays on, it could just be a malfunction of the light. To reset it, disconnect the battery for 10 minutes.
Take the master cylinder off and then on the inside of the firewall where the rod from the brake pedal goes through, there should be 4 nuts holding the booster on.
A 1987 s10 Blazer 4x4 weights around 10.6. This is going off curb weight.
The power brake vacuum booster is leaking, replace it.
Check the fuse, then the bulbs, then the brake light switch.
The library should have a shop manual...