So, just seeing if this is a common thing - was driving in pretty hot weather (Utah) around 100deg F for 3 hours in semi atrocious traffic ie low speed, a lot of stopping, congested interstate parking lot type stuff, but managed to keep moving most of time driving. Juuust before getting home had “Brake Failure” light and mushy pedal. Reservoir was full, didnt see any leaks. Any suggestions on what to start checking? 1988 Volvo 245 DL Auto B230F

 

Master cylinder (internal leak) and vacuum line in brake booster. Check all brake calipers for leaks.

 

would also mention; when not running, brake pressure was ok and firm as usual. would prolonged running/heat have any effect on the brake booster or vacuum?

 

any under the hood noises when pressing the pedal (with car running)?

 

pre-ABS 240's have dual circuits, and the brake warning light is telling you they are unbalanced. one circuit goes to half of each front brake, and one of the rear brakes, the other circuit goes to the other half of both fronts, and the other rear. its designed this way from a safety standpoint, even if a circuit fails entirely, you still have 50% braking force on both front wheels....

you need to bleed all 6 slave circuits.

 

When was the brake fluid last changed? Old fluid sucks moisture (water) which boils in heated conditions. I'd start by changing it with good quality fluid, bled as needed.

 

I had a problem of losing a little vacuum when I had been on the highway at high speed and didn't tap the brakes for a long time. My mechanic said it's normal to lose a little vacuum in those circumstances. The master cylinder, booster and vacuum lines all checked OK. For now, I believe the mechanic.

 

is that a turbo? turbos under moderately heavy throttle tend not to have any vacuum, but most any other car will, and there's a 1-way valve in the vacuum connection to the brake booster which SHOULD allow the vacuum to be held for quite a long time.

I remember pumping my brakes repeatedly on my stopped 240 after it had been shut down for a half hour, and I could feel the vacuum booster for the first 6-8 brake depressions, then finally it would lose vacuum, and the pedal would become stiffer and harder to press. This was on a well maintained car with 350k or 400k miles.

I don't ever remember losing brake vacuum on my 740T wagon, even after ripping up I80 from east of Sacramento to the top of Donner Summit at extra-legal speeds with a heavy load, and bikes+cargo box on the roof, the pedal was still powered and fine, and I know I was under heavy boost the whole time.




so, if its losing power brake assist, I'd be checking the vacuum hoses and that one-way valve.