My brake master cylinder is kaput. It regurgitated brake fluid into the power brake booster.
Should I replace the booster as well as the master cylinder, or does it matter?
Does the brake booster still work? If you had a Mityvac hand pump you could apply vacuum with that to see if the booster holds vacuum.
If it does and money is tight and you just need to get the vehicle back on the road (with a plan to replace the booster later) you could clean up the brake fluid inside the cupped area at the center front of the booster and install your new Master Cylinder and then test the Booster again to see if it holds vacuum.
There is a cupped area on the booster (behind the MC) where the pushrod and seal are located and if brake fluid did not get past the pushrod seal you might be OK (just make sure your Emergency brakes work). So first clean out the area around the booster pushrod, if you have some lithium base SILICONE grease (not petroleum base wheel bearing grease) you can put a smear on the pushrod (have someone press on the brake pedal to extend the pushrod while you clean the pushrod and put the silicone grease on the rod and seal). Example of one type of grease that can be used IME is Molykote 33 Medium (it doesn't have "Moly" in it despite the brand name, McMaster-Carr carries it).
After cleaning and greasing the pushrod and pushrod seal and replacing the Master Cylinder you can do some standard booster tests ie:
Engine off, pump the brake pedal until it's firm
Hold pressure (down) on the pedal and start the engine
The pedal should drop slightly as the vacuum assist comes on
But if that leak has been there awhile and as much as shown in the photo then brake fluid could have migrated into the booster
and that can lead to failure of the booster as mentioned by others above. Doing both at the same time makes sense especially if
the booster is the original.
