A brake "relay" would probably be used in an ABS system. That needs to be checked out by a qualified mechanic that is familiar with the ABS brake system.
You may want to start with the brake fuse. If you still have turn signals and lights. you may also want to check the brake electical relay switch in the relay box Check as well the brake switch located at the brake pedal.
Im not sure what you mean by RELAY brake light switch. There is 1 brake light switch on the brake pedal. Check it with an OHM meter,wires disconnected brake off should be "open" (no connection) press the pedal and should have no resistance (0 OHMS)
check the brake light relay under the hood
Then fix them. Start by checking the bulbs - those are dual filament bulbs, and it's possible that the filament for the brake lights was compromised. If those check out, see if you have power to the brake light circuit with the brake pedal depressed by using a test light. If you don't have power, what I'd do next is find the relay and test that. Remove the relay, check for power at the 30 post - it should be hot constantly. Then check for power at the 86 post with the brake pedal depressed. If you have power at the 86 pole with the brake pedal depressed and at the 30 pole, then the problem is either the relay itself, or the wiring along the 87 circuit. If you don't have power at the 30 pole, check to see if there's a fuse specifically for the brake lights. If you don't have power at the 86 pole with the brake pedal depressed, the problem is somewhere between the brake pedal and the relay. I'd start with the brake light switch itself if that were the case. Should be located at the top of the brake pedal.
The brake relay switch can be found on the firewall in the engine compartment. The brake relay switch will be on the drivers side of the firewall.
It should have only a fuse and a switch, no relay. Check ALL of the fuses with a test light.
The 1991 Volvo brake light relay switch is located beneath the drivers side dashboard. The brake light relay switch will be above the brake pedal.
There is no brake light relay. There is however a brake light switch located above the brake pedal.
First off the quit making the 780 in 1990. What do you mean brake relay? Brake light relay/integrity sensor? Need more information
The way i understand it is that the relay has the hot and the ground connected and when you press the brake the relay works like a toggle and completes the circuit.
Turn Signal / Brake Light Troubleshooting (check in this order): 1. Check Fuse 2. Check Bulbs 3. Check Flasher Relay 4. Check Turn Signal Switch in steering column and/or Brake switch down by brake pedal. Most likely its the fuse or bulb. See sources and related links below for bulb information.
Check the brake light fuse first. There is a switch that controls the brake lights. Some are mechanical, located on the brake pedal lever, and some are pressure, located on the master cylinder. If it is mechanical, it may just need to be adjusted. After checking fuse, brake pedal swith, it's probably the Bulb failure warning light relay, a big round and red relay locate close the fuses box, back the astray, close the transmission shifter. Check FCP Groton use Bobby1 as a coupon. Good luck