Bleed the brakes. You've got air in the left brake line.

Does this happen every time you apply the brakes or is it only
sometimes?  Does it only happen after you have just backed up a ways?

My guess is worn bushings on the front suspension.

Hi Ged

I've had this problem on two BMW's of similar ages to yours, each time
it was due to wear on the front discs. As they wear down, a shoulder is
left on the largest diameter of the discs and so when the brakes are
applied, the pads - or sometimes the metal backing plate of the discs,
contacts with this shoulder first which causes some inbalance but as
the pads met the discs they even up again. To cure this you can either
take off the shoulder with an angle grinder, or if they're too far gone
then replace them. If your're in the UK you can buy ATE discs from Euro
car parts at a much better price than BMW and they're the same thing!

Good luck
Tony

My vote (not knowing how many miles your car has on it) would be
suspension bushings.
--
C.R. Krieger
(Been there; done that)