I recently had a caliper replaced on the front driver’s side wheel. I had driven it a bit at highway speeds before I realized it needed to be replaced.
Now, the car drives fine while cold or at normal temperatures. The problem will start after heavy use of the brakes, like when my wife is driving (I down shift) or going downhill. Once the disks heat up the steering starts to build a noticeable wobble. The more brake usage the more wobble and if I lay off the brakes so they cool off a bit the wobble will go away.
I figure one of the front disks is warping as they heat. Would this be a good assumption?
If so, what is a good way to figure out which side is the culprit without having to change the rotor twice if I get it wrong the first time?
Oh, you mean . . brakes.
When it comes to brakes, you don’t replaced just one rotor. You do a complete brake job.
That means you replace both rotors and the brake pads.
Tester
How old are the rotors ?
Have they been ‘‘turned’’ before ? aka re-surfaced ?
Measure , or have them measured, with a caliper to check for minimum thickness. This number is cast into the rotor usually on the backside or outer edge.
That minimum thickness is important and even if they’re above , but near minimum, it could be the cause of heat warp.
Front vs rear can be differentiated usually by using the parking brake as a test. The parking brake only affects the rear brakes on most cars. Doing that you could confirm this is a front brake problem. That’s something to go on at least.
If it confirms the problem is the front brakes, one idea is the work done to replace the caliper meant the wheel had to be taken off. If the wheel was put back on incorrectly – lug nuts over tightened or improperly sequenced – this symptom could result. If I had this problem on my Corolla besides a visual inspection, first thing I’d do, I’d jack up the front of the car, jackstands, remove both front wheels and reinstall them, making sure to torque the lug nuts in the proper sequencing and torque increments.
If you replaced the caliper, it was most likely because it was binding. This binding heated up the rotor and now it is warped.
As others have mentioned rotors and pads should be replaced in pairs.
I would replace rotors and pads on the front. This should solve your problem, but I would advise that you check to be sure that this caliper is not still binding up. Many times it is not the caliper that is causing the binding, but rather the flexible brake line to that caliper. These lines can deteriorate from the inside and start to break down. The fragments if rubber hose can act as a check valve…allowing the fluid to pass to push the caliper piston out, but not allow that fluid to return and release the caliper.
If it is the hose and you replace the rotors first, you will just damage the new rotors before you find that it was the hose causing the problems.
Yosemite
@Tester is correct… if and when you discovered a failing brake caliper…my guess is that it was binding up on you…this overheats and warps the brake rotors. This warpage gets more noticeable when you put some significant heat into the rotor upon braking…it will not go away.
If I was doing this job…since one caliper has already died on you…I would replace the other brake caliper and also BOTH brake rotors.
Buy quality Rotors!!! I cannot stress this enough…there are severe quality issues with rotors on the market today. You can buy them from the dealership (expensive), but you should be OK if you use the rotors that have a 3 yr warranty against warpage (Auto Zone) comes to mind. They aren’t very expensive ($50-ish ea) and the warranty is protection enough for me. Their 3yr rotors are also coated with an anti rust treatment…I haven’t seen many problems with them…and if I do…I will be sure to have those problems before the warranty expires. 
SO…replace the other caliper…because it will fail soon enough…you don’t want it to fail with new rotors…or you will warp a brand new rotor. Replace the other caliper, the pads and Both front rotors and you will put this issue to bed post haste.
Blackbird
The parking brake is no longer a good test for rear brake rotor warpage in many cars because it’s become more common to use a tiny drum brake inside the disc as the parking brake. It doesn’t actually use the service calipers at all, so all you’re doing by pulling the e-brake at speed is wearing down the very thin brake shoe on the emergency brake – and it’s only good for about 6 pulls if that, so you don’t want to use it unless it’s an actual emergency.
If you take it to a mechanic they will always replace both rotors no matter what. I don’t always agree that this is necessary - it depends on how old they are. If you’ve got 100k on the rotors, then yes, replace them both. If you’ve only got 10k on them, there’s no need to toss out a perfectly good rotor just because the other one is damaged.
Also, can we stop calling people out over “breaks” vs “brakes?” We all know what the OPs mean, and while I fully acknowledge that misspellings can be annoying to those of us who like the English language, this is after all Cartalk, not Literacytalk. Just spell it correctly in your answer and OP will probably get the hint.
@shadowfax makes an excellent point… Many vehicles are using those dual brake type of setup…its a disc brake rotor with a small drum in the middle with little brake shoes handling P Brake duties.
It also Brakes my heart when the Breaks on my car, well…Brake… I also agree to give the OP’s a Brake when they have Braking news about their vehicles Breaks…Braking.
Please dont try to correct my Break usage…if you couldn’t tell…it was intentional. Hahaha
Blackbird
The OPs car has drum brakes in the rear so any warped rotor is up front.
There was a “performance brake” option that included ABS which replaced the rear drums with discs.
Thank you for the input! Sorry about the misspelling. It has been fixed. 
It is 4 wheel disk. The e-brakes use the back wheels, and the peddle brakes use the front mainly.
The dealership replaced the caliper and said the pads were fine.
Thanks again!
You seem to think that because your emergency (parking) brake works on the rear wheels, the brake pedal doesn’t. Your brake pedal works on all 4 wheels whether you have rear disc or drums.
There were few 2004 PTs with rear disc brakes. They were mostly the 220 hp GTs. If you have alloy wheels, look through the spokes to see if you see a shiny disc surface like in the front or if you see s rusty drum.