Help! Sudden brake failure during test drive
 

While taking the project out for a test drive after changing the MAF and Coilpacks/plugs, everything ran great. Drove 20 minutes on the interstate to my father's house and about 10 minutes into the test drive with him the brake pedal went to the floor suddenly, without warning.

I had to pump the brakes several times to get it to stop and they never felt strong. After we limped it back to his house there were no visible leaks, definitely smelled brake fluid, but the levels stay solid even after pumping the brakes a while. They feel normal again now though, going to try a drive on the street its parked on just to see if all the braking power is back.

Im just looking for other people's thoughts. Seemed like a master cylinder from the behaviour driving, and it does seem to maybe very very slowly continue to sink down after I start the car with pressure on the pedal. Went ahead and ordered one but we also noticed a clinking/clunking sound when hitting the brakes at the front right caliper.

Caliper behavior and the seeming return of brake pressure has me thinking the caliper may be engaged slightly and caused the fluid to boil, that would explain the smell with no leaks right? Didn't notice any pulling to one side at the time though or any grinding noise but it does seem reluctant to roll in neutral with a good heavy push.

Oh and while inspecting the MC with and hitting the brake pedal I heard like an air swooshing sound coming from the MC/Brake booster. Is that just the sound a brake booster makes when pushing the cylinder?

update info: 1999 boxster, has sat for a while before I started bringing it back to life

Long shot, but when was the last time the brake fluid was changed and the system bled? Before I would replace anything, a proper brake fluid change and meticulous bleeding would be the order of the day...
In addition, the system must of course be checked for leaks, but this would be noticed during the change.

Last night I did flush the system with new fluid but still have some air in the system after 2 bleeding sessions(we didn't do the inside bleeds) going to bleed again tonight.

But we did also get the car in the air and in nuetral and confirmed that the front right wheel is hard to move and the caliper might be partially engaged or seized. Im thinking the partially engaged caliper overheated the old fluid and boiled the brakes. The brakes felt fine last night on a quick test drive on the street before we flushed with new fluid.

The current plan is to change the master cylinder(might as well idk when it was changed) and rebuild the calipers. Want to try to get it back to the house tonight though if we can get it bled all the way. Going to take off caliper tonight as well and just pull the pads and have a look at the pistons/clean the pad tracks.

I chased the same problem, turned out to be the ABS unit.

How did the problem manifest itself for you?

Brakes went to the floor.

Update
 

Got the car back home. After flushing with fluid and bleeding the brakes the brakes worked fine. Was able to confirm while car was in neutral that the front right caliper is partially engaged. New fluid prevented a fluid boil on the drive home. Still going to replace MC along with the Caliper rebuild cause why not.

But if you l live in a humid climate and driving an old boxster that's been sitting, flush the brake fluid first! 95% positive the fluid boiled from being too old and getting heated quickly by the engaged caliper.

So I guess you also confirmed that the brake is sticking enough to get really hot? It should be very noticeable keeping your hand close to the wheel. You should also see the disk having a purple color.

Maybe I haven't ever driven with really bad brake fluid, but while I have experienced brake fade due to a sticking caliper (and pulling of the car as a consequence) it didn't result in loss of brake pressure. Once one of my cars stood there with a smoking brake, still no loss of pressure.

Was able to confirm the front right wheel got much hotter compared to the front left and rears. Since driving with new fluid I've had no issues. Also using some brake cleaner on the front right caliper along with working the pedal was able to release the partially engaged caliper. Still going to rebuild them for peace of mind and replace the MC with the new one when I do the rebuild.