I took my 98 S40 with 160,000 miles (I know its a lot!) into my local
garage yesterday as my break pedal went hard when driving.

Volvo (who I took it to first) thought it may be a locked break
caliper but the garage checked it over and told me there was an air
leak from the vacuum hose and that was causing the pedal to get hard
and making the idle on my car drop down to around 500 whenever I
slowed down for traffic or lights.

So they changed it and I picked my car up and drove off and the pedal
was soft for a few minutes then went hard and then within 15 minutes
my revs were dropping to 300-400 RPM and then I started stalling!

I must have stalled 10 times - everytime I slowed down - although when
I got on the main road with no traffic the revs seems to be idling at
800ish.

So what is going on? my car was NEVER stalling before they changed the
vacuum hose!

and when I have been driving fast for 20 miles it seems to be idleing
normally? but if I was sitting in traffic I am sure it would start
stalling!

The garage didnt know what it was and said they think there is
probably another air leak somewhere causing it!

Does anyone have any ideas what it may be? thanks.