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Replying to BrickMan here, but thanks everyone for all for the replies!
Had to google runout and do I understand it correctly that it's the disc is either mounted slightly on an angle due to rust / debris / break dust buildup or poorly manufactured parts? This then causes the discs to thin in places, which slowly builds over time creating the pulsing while braking?
My tires are new this summer, just a few months old and there's no vibration even at higher speeds so everything seems to be balanced and trued properly. All discs and pads were changed in sept last year and the car has prob seen 700-1200 miles since then.
As it hasn't been a year since the discs were changed I'm gonna give the garage who changed them a call
@lynx Car is a 2007 Saab 9-5, the judder started a few months ago but has been getting rapidly worse over the last couple of weeks
update - just spoke to mechanic who's gonna speak to their parts supplier about warranties. Fingers crossed they'll sort it out for me.
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kboy
Run out between the hub and inner face of disc. Only takes a tiny amount (0.2mm) to cause epic problems, usually around 1000-2000 miles from install.
Disc unlikely to be warped, more common is disc thinning or just plain runout.
Runout you can maybe fix by taking it all apart and sorting the run out with a dial gauge, but usually by the time you have the symptoms is too late for that disc, as one area (high spot) will have been thinned down minutely by hundreds of thousands of passes across disc pad.
I went through three sets of discs on one of mine until I sorted it.
First time, my bad, I had 10x the permissable runout at the hub (read the gauge wrong)
Second time run out was at limit of spec but couldn't make hub any cleaner, so bought new hubs (face was bent from either over tightened or road damage). Didn't get dial gauge our, just whacked my new £90 a corner disc on my new £170 a corner hub assuming both German companies who made them, made them right.
Third time. One hub was not straight from factory and caused thinning of a disc, again. Replace hub, again, replace did for third time, and finally no problems.
During this time also.... Rebuilt brake calipers and master with all new internals to irraicate chance of a slow returning piston from causing disc thinning and checked every bush and joint in the car about 8 times.
Cars. Yeay.