Hi everyone! Bit of a technical one here and down to it being the brakes, a pretty important one!

Basically, when driving, the brake pedal will have a soft, extremely assisted feel to it - with a fair bit of progression - you press down and there's instant bite, with the brakes clamping solid until the ABS kicks in if required.

However, seemingly at random, the brake pedal will go solid as a rock and all assistance will evaporate into thin air - and it's not fun trying to stop the car like that because unassisted braking effort is about as good as breathing in the direction of travel!

What's most likely to cause this? My thoughts are aimed at the brake booster, because that's what seems to be the issue - but it's a shiny brand new one (still in warranty too!).

I thought maybe the master cylinder could be playing up, leaking fluid onto the booster, but I'd expect it just to fail altogether, not be intermittent and seemingly random.

Also, the brake warning light has begun illuminating, seemingly at random, even when not using the brakes - but it's usually for a very, very brief period of time.

The car has ABS, which self-tests properly and works A-OK.

So, anyone know what could cause intermittent assistance failure? I've not really dealt with braking systems as involved as this one before!

Could it be a leak from a damaged vacuum line or something? I haven't had the chance to look properly yet but IIRC the underbonnet stuff is in good order

I've read around on here and come to the possibility that it might be a duff booster, with the diaphram not returning to the correct position unless poked about? Any opinion on that?

Thanks

Don't let me resort to rocket brakes!

 

If it is any consolation, my 1987 has the same exact issue. Some days she works fine, then other days it seems the brake pedal sits higher or requires more/less pressure to operate. May be master cylinder.

 

Master failure will make pedal sink and feel soft. Look at the check valve for the booster, make sure it works proper. Also inspect the vacuum hose to make sure its not cracked, pours or collapsing during periods of higher vacuum.