Right and left brake lights do not work when brake pedal depressed. Both tail lamps work as well as both turn signals

FUSES BAD OR MODULES AND SWITCHES
NO BCM brain box here.
USA car , canada is NO THE SAME electrics !!!!
so flashers means turns, and turns are dead all 4 corners or what? all flasters is HAZARD lamps switch
parks work.(tails and front parks)
turns fail all 4 corners(not told) and hazards dead too all four corners and brake lamps all 3 rear dead.
do not break the brake lamps.
grounds to all lamps good,(the failed ones)
0vdc on ground.
fuses are always first, a 20cent fix. is way and childs play test (or a look)
fuse 1 and 9 not blown in IP BOX x2 , 15amps
OMG fuse 9 runs brake lamps. is this clue , YES
the next in LINE is Electronic flaster not bad.
5 wires and the black ground wire not busted.G204
next in line is the very complex MULTIswitch (column)
next is the BRAKE PEDAL switch NOT BAD.
if not bad 12vdc (push brake pedal) leaves the switch
R/GL wire to all brake lamps including 3 center high mounted lamp and its G300 ground

fix brake lamps only first.
fix that and others get fixed for free (shirt tails free)
to test lamps at rear for brakes the pedal must be depressed . a 2 man job.
fuse 9, hot both sides. and is hot all the time bat side.
brake switch LG/R wire hot all the time key off too.
light green red !
step on brake R/LG goes hot, now. if not switch is bad
Red/light green. (means)

What's a DHS ?
Stop Lamps

The STOP LP fuse in the rear fuse block supplies battery positive voltage to the normally open stop lamp switch. When the brake pedal is pressed, the stop lamp switch contacts close and battery positive voltage is supplied to the center high mounted stop lamp (CHMSL) and the turn signal/hazard flasher module stop lamp switch signal input. In response to this input the turn signal/hazard flasher module supplies battery positive voltage to the left and right LED tail lamp assemblies. The CHMSL and left tail lamp have ground at G402. The right tail lamp has ground at G401. The stop lamps illuminate when they receive both power and ground.
es battery positive voltage to the left and right LED tail lamp assemblies. The CHMSL and left tail lamp have ground at G402. The right tail lamp has ground at G401. The stop lamps illuminate when they receive both power and ground.

Stop Lamps
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The body control module (BCM) provides a 5-volt reference voltage and low reference to the brake pedal position sensor. The position sensor provides an analog voltage signal back to the BCM that will increase as the brake pedal is applied. When the brake is applied, the BCM will then apply battery voltage to the left and right stop/turn signal lamp supply voltage circuits and the center high mounted stop lamp (CHMSL) supply voltage circuit illuminating the stop lamps. At the same time the BCM applies battery voltage through the signal circuit to the transmission control module (TCM) and the engine control module (ECM) indicating that the brakes are applied.
Ground for the stop lamps is supplied as follows:
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps
• G301 provides ground for the CHMSL
The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.
Battery voltage from the LT TRN/SIG and RT TRN/SIG fuses located in the rear fuse block is also used by the body control module (BCM) for stop lamp operation.

Turn Signal Lamps
Battery voltage supplied to the body control module (BCM) from the fuses listed below is used for turn signal and hazard lamp operation.
• LT TRN/SIG fuse for the left turn signals
• CTSY/RF TRN fuse for the right front turn signal
• RR TRN/SIG fuse for the right rear turn signal
When the turn signal switch is place in either the LEFT or RIGHT position, ground from G300 is applied through the turn signal switch signal circuit to the BCM indicating the turn signal request. In response to this signal, the BCM applies a pulsating voltage to the front and rear turn signal lamps supply voltage circuits cycling the lamps ON and OFF. At the same time the BCM sends a GMLAN serial data message to the instrument panel cluster (IPC) to cycle the appropriate turn signal indicator ON and OFF.
Ground for the turn signal lamps is supplied as follows:
• G104 provides ground for the left and right front turn signal lamps
• G402 provides ground for the left rear stop/tail/turn signal lamps
• G403 provides ground for the right rear stop/tail/turn signal lamps

Do you have a factory scan tool (tech2) ? Your best bet , take it to the dealer .

Kevin , this is a very complex way of stoplamp control , I will post GM service Description and Operation of how they work an let you deside if you want to tackle this ! Do the center high mount brake lights work ?
The brake pedal position sensor is a resistive device used to sense the action of the driver's application of the brake pedal. The brake pedal position sensor provides an analog signal that will increase as the brake pedal is pressed.
The brake pedal position sensor receives both a 5-volt reference voltage and a low reference signal from the electronic brake control module (EBCM). When the brake pedal is pressed, a variable signal is applied from the brake pedal position sensor to the EBCM through the brake sensor signal circuit. The fuse block left rear supplies battery positive voltage to the bass relay switch through the BASS fuse. One side of the bass relay coil is permanently grounded at G402. The EBCM supplies voltage to the bass relay coil upon receiving the signal from the brake pedal position sensor. The switch contacts of the bas relay close and battery positive voltage is applied through the bas relay switch to the center high mounted stop lamp (CHMSL), and to the turn signal/hazard flasher module through the stop lamp switch signal circuit. The ground for the CHMSL is applied from G402. The turn signal/hazard flasher module then supplies voltage to the left and right stop lamps in the tail lamp assemblies that have ground at G401 and G402. For export vehicles the stop lamps receive battery positive voltage directly from the bass relay.

There could be a code stored in the EBCM - electronic brake control module - DTC B3903 Stop Lamp Relay Circuit
DTC C0278 Brake Pedal Position Sensor Not Calibrated
there are several other codes that pertane to the brake lights . you could try Replaceing the bas relay !
Fuse Block - Left Rear
In the passenger compartment, under the left side of the rear seat at carpet.

Check the brake light switch on the brake pedal. Sounds like it slipped out of adjustment. When pedal is depressed, the button on switch will extend and be on to pass current to the bulbs.
Also, you need to figure out the tail lights: those two filament bulbs are for park lights and for brakes/turn signals. One filament is for the parks, the other filament is for both brake and turn signals. American cars have the brake lights wired through the turn signal switch- from the brake light switch on the brake pedal to the turn switch, and then out to the rear bulbs for brakes and, or turn signals.

Turn Signal Lamps
The LT T/SIG and RT T/SIG fuses located in the underhood fuse block supplies battery positive voltage to the body control module (BCM) for turn signal and hazard lamp operation. When the turn signal switch is place in either the LEFT or RIGHT position, ground from G202 is applied through the turn signal switch signal circuit to the BCM indicating the turn signal request. In response to this signal, the BCM applies a pulsating voltage to the front and rear turn signal lamps supply voltage circuits cycling the lamps ON and OFF. The BCM also sends a message via GMLAN to the instrument panel cluster (IPC) to cycle the turn signal indicator ON and OFF depending on the position of the turn signal switch.
Ground for the turn signal lamps is supplied as follows:
• G101 provides ground for the left front turn signal lamp
• G100 provides ground for the right front turn signal lamp
• G302 provides ground for the left and right rear turn signal lamps

The LT T/SIG and RT T/SIG fuses located in the underhood fuse block are also used by the body control module (BCM) for stop lamp operation.
The brake pedal position sensor is used to sense the action of the driver application of the brake pedal. The body control module (BCM) provides both, a 5-volt reference voltage and low reference to the brake pedal position sensor. The position sensor provides an analog voltage signal back to the BCM that will increase as the brake pedal is applied. When the brake pedal is applied, the BCM will then apply battery voltage to the left and right stop lamp supply voltage circuits and the center high mounted stop lamp (CHMSL) supply voltage circuit illuminating the stop lamps. At the same time the BCM signals the transmission control module (TCM) and the engine control module (ECM) that the brakes are applied. Ground for all stop lamps is provided at G302.
The stop lamps on this vehicle will not illuminate unless the ignition is in the accessory, run, or crank positions. When the ignition is in the OFF position the stop lamps will not illuminate when the brake pedal is applied.

Did you check the fuse's. ? Wouldn't think it's a ground because right an left tail lamp Assembly's use same ground ! Did you check for B+ voltage at the bulb. with a volt meter ? Power is supplied by the BCM . Could be the tail lamp assembly

It could be a bad multifunction (turn signal) switch. The way that switch works, it is possible that it can turn on the front blinker and not the rear one. It also cancels the brake circuit so the turn signal can work with the brake pedal depressed.

However, a more likely cause would be a break in the YELLOW/BROWN wire at the rear of the truck. It is common for the connector to get corroded at the left rear frame area where the bed harness connects to the vehicle harness. If you have trailer connections spliced into the wiring harness, these connections are also prime candidates for corrosion. I would look there first. Inspect these harnesses thoroughly all the way up and into the tail light assembly.

If the wires under the back of the truck look good, then I would check for power coming out of the turn signal switch. The wire at the turn signal switch is plain YELLOW (with no brown tracer). It is Pin (M) on the turn signal switch connector. The front signal is a LIGHT BLUE wire at the turn signal switch Pin (H). If you probe these with a test lamp, they should both work the same except the yellow wire will be hot with the brake pedal depressed and the switch NOT in the left turn position. If there is no power at the switch connector, replace the switch.

I had similar symptoms - rapid flashing of directional when headlights on and brake depressed when making a left turn, both directional indicators in dash light dimly when brake depressed, "Check Traction Control System" indicator On even with ATC turned Off and "strange" indications in tail lights.

Check the pins in the connector at the Lamp Holder Assembly for your Rt. Tail Light for arcing and corrosion. (The female pin is in the vehicle harness connector and the male is part of the Lamp Holder Assembly.) If found, because the male connector is part of the Lamp Holder Assembly, you'll have to replace the Lamp Holder Assembly and the harness section coming in from the vehicle containing the female connector.

I just repaired this problem in the left tail light on my '01 van with a part from a '99 van I found at a local salvage yard.

the upper brake light works differently than the outer lights. check your brake light switch by the brake pedal.It will have a plunger switch on it. The center brake lamp works off a power convertor instead off the brake light switch. The switch has a separate wire for that which is still working

no disrespect but....
Check the book and make sure you are using the proper bulbs on all sides. (3957 looks just like 3956 but wont work right)