Intermittent is hard to explain. I'd have thought air in the system would be there all the time and have a constant effect. Likewise a ballooning hose.

One possibility would be boiling brake fluid, which would be intermittent depending on heat input. This would be resolved by changing the brake fluid.

Another might be a possibly damaged seal hanging up or everting occaisionally. This would require a rebuild, and might be hard to find

A third might be a caliper slider jammed most of the time but occaisionally slipping. This would be expected to give you uneven and compromised braking most of the time

Edited by edlithgow on 23/02/2024 at 00:57

Update.

Car was investigated by local mechanic. Brake fluid checked and found to be ok.

Rear brake drums and shoes stripped and adjusted.

They couldn't replicate the problem but it is still an issue.

Earlier today when driving, pedal went soft again and I *think* the ABS kicked in which suggests this is where the problem lies.

Garage said either ABS unit or master cylinder.

Question: how much to replace and will it need recoding to car?