The prerequisite for this course is "Lighting Gremlins" ET-103, where you learn all about checking and fixing power and ground connections in the lighting section of the wiring harness. The most important point of that exercise is to assure that you have good ground connections on all of the lamp housings before even attempting to diagnose anything else. This course continues on from there to discuss the switched and fused circuits for brake lights and turn signals.
I trust you have a 1500 car with single rear lamp fixtures and the turn signal relay box. For brake lights and turn signals, start with this diagram and print for reference:
MGA switched and fused lighting circuits, 1500 "circ_f2.htm".
".... I get good readings at .... the turn indicator switch (green, green yellow, green blue wires)."
For testing and debugging the brake lights and turn signals you need the ignition switch turned on. If you leave the ignition on for more than a few minutes at a time without the engine running, you run a risk of burning out the ignition coil. So before you go into this routine, disconnect one primary wire from the ignition coil, either the hot white or the white/black going to the distributor.
The first thing you should notice in the wiring diagram is that the turn signals and brake lights are sharing the ground connections with the parking lights and tail lights. So be sure the tail lights work (prior course) before going here.
You should also notice that the brake lights and rear turn signals on the MGA 1500 share the same high intensity filament in the single bulb in the rear lamps. This is what makes the big turn signal relay box necessary. When you switch on a turn signal, the relay disconnects the rear lamp from the brake light circuit and connects it to the front lamp and to the flasher unit with a 3-way relay connection. So before you go into the turn signals, you want to test the brake lights.
When the tail lights work (checking first for good ground), then switch on the key and step on the brake pedal to see if the brake lights work. If you get no brake lights, then check for power at the brake light switch. This you will find on top of the frame below the starter switch. The green wire should be hot with the switch on. If not, then back up one connector in the harness and look for power at the bullet connector in the engine bay, a 2-green-wire connector at same location as the 3-red-wire connector for the parking lights, near the starter switch. Go no farther until you have power at the brake light switch.
With power at the brake light switch, step on the pedal again. If you still have no brake lights you may have a bad hydraulic switch. Connect a jumper wire across the brake light switch to bypass it. If the brake lights work, then you need to replace the switch.
If the brake lights still don't work, then leave the power jumper connected across the switch and move on to the next connector in the harness. Find the 2-wire green/purple bullet connector near the starter switch. Check for power there which will now be coming from the brake light switch jumper. Clean the contacts if necessary to assure good connections at this location. Check again to see if you have brake lights.
If still no brake lights, then go to the next junction for that circuit, the green/purple wire on terminal 5 of the turn signal relay. Check for power at this terminal. If no power here, back up one paragraph, because you missed something. If you should ever find power at one end of a wire and no power at the other end of the same wire, the you have a broken wire (which is virtually impossible unless the harness is burned).
Once you have power on terminal 5 of the relay, the brake lights should be lit. If not, immediately check for power on terminals 3 and 7. If power on 5 but not on BOTH 3 and 7, then you have a bad normally closed contact in the relay. Then you may get your first look inside the relay. Or you may have to do this later for other reasons anyway. With the cover removed from the relay you can use emery paper to clean the contacts, as with any typical relay contacts.