Topic

Hi all. My 97 MGF has developed a strange brake issue that I'm having trouble diagnosing.
When I push the brake pedal, it goes long, about half way to the floor. The brakes work but don't instil confidence. If I pump the pedal, the pedal stays high, where it should be, and feels rock solid, no sponginess at all. I start driving again and next brake application, the pedal is low again until pumped and again it comes up and feels solid. Brake system is factory standard with new rotors and pads all round a thousand Kms old.
Here's what I've done. Bled whole system twice or three times, replaced brake master cylinder, replaced servo, removed, inspected and refitted all calipers, no seized pistons or seized pads, lubed all caliper slides. Clamping off all flexible pipes one at a time didn't change the symptom (car was in the air not driven).
So the system appears to be in good order yet still this long pedal at first press.
I am unfamiliar with what kind of problems you get with ABS modulators. I've worked on my own cars for 40 years but this is the first with ABS. There's no obvious way to bleed the modulator.

If anyone else has experienced this issue and can offer any insight, it would be greatly appreciated.

Thanks in advance. Scotty.

Hello Scotty,
You have seemed to have tried everything, the ABS pump should be self bleeding when you pull fluid through.
How are you bleeding? Easy bleed or some other way, i find the way that works for me is to have someone in the car pumping the pedal and holding pressure while you start at the back , the extra pressure helps to force trapped air out.
The symptoms you have are synonymous with air in the system.
All the best.

If you have not used a vaccum bleeder to bleed the brakes, do so. The ABS system can hold air in the valve block that is difficult to dislodge any other way.

How recently were the flexi lines replaced? They go soft with age and tend to expand reducing pressure to the calipers.

Thanks. I wondered if there might be an air lock in the modulator. I did bleed the system the usual way, tube, glass jar, spanner and a helper. I'll investigate vacuum bleeding. Its worth a try.

Flexi hoses weren't replaced but inspecting each when brakes applied don't show any that are bulging or appear to be deteriorating, but if I can't find what's causing the problem I'll have to replace them just to eliminate tham.

Thanks to everyone that has replied so far. I didn't think that there could be air in the system as when the pedal is pumped, it is solid, not spongy, but I could be getting misled.
I forgot to mention in my original post that when I apply the brakes, the pedal goes down slightly & the brakes slow the car down, but continued pressure makes the pedal sink to about half way over 2 or 3 seconds. My original message suggested that the pedal went straight down to half way.

Also, apologies for delay in replying, but when most of you are on line, I'm asleep, courtesy of being down under.

Again thanks for the help.
Scotty

That is a fault in the master cylinder. Try slackening the outlets, one at a time and bleed it through in case one side of it has air in.