Technical Intermittent brake issue...

[strudelpus]

Hi folks,
Brake pedal sporadically really spongy going to the floor, but immediate re-application has the normal firm pedal feel. Servo vacuum hose looks fine and no apparent leaks/fluid loss. Wondering if an air bubble is floating around the system or an issue with master cylinder internals. Perplexing intermittent fault...

 

[koalar]

fix immediately

don't drive the car

 

[koalar]

nothing to do with the servo or vacuum side


its hydraulic


air is mostly entering the system via a weeping cylinder, calliper or master


you could try bleeding to see if any bubbles point to corner of the car.

 

[Pugglt Auld Jock]

nothing to do with the servo or vacuum side


its hydraulic


air is mostly entering the system via a weeping cylinder, calliper or master


you could try bleeding to see if any bubbles point to corner of the car.

Most likely to be master cylinder from the limited evidence given here? A damaged seal, or especially one which is reversing itself (not common but I have had experience of one doing this) could give this.

 

[koalar]

Most likely to be master cylinder from the limited evidence given here? A damaged seal, or especially one which is reversing itself (not common but I have had experience of one doing this) could give this.

that would be my guess


but it best to check everything.

 

[koalar]

you can boil the fluid if one of the brakes is dragging but you should be able to smell it


or its not failing on long descents with hairpin bends or when driven like you stole it ?

 

[timmycm850]

Hi

If the pedal goes down all the way when you press very gently and locks at the top when you stamp on the pedal then its the master cylinder, based upon your description.

Tim

 

[strudelpus]

If the pedal goes down all the way when you press very gently and locks at the top when you stamp on the pedal then its the master cylinder

It doesn't so much Lock at the Top, more immediately reverts to normal operation, but thinking about it i agree that it may well be an internal master/cyl seal folding over and reseating...
Thanks for the input, folks!

 

[koalar]

10 minute job thats worth checking


Take the rear drums off and check the shoes linings haven't fallen off


normal they lock on when reversing but as they move around the pedal feels different.


Its fairly common with the original Bosch assuming is not a rear disc model


while there its worth checking behind the rubber seals for weeping. A small amount will not be the problem but will need changing

 

[DaveMcT]

Check the brakes and hoses, especially rear drums. If they are all OK then it's likely to be the master cylinder.

The plastic covering on the metal pipes tends to expand under the flare nuts. Warm them with a gas lighter flame and they'll unscrew without twisting the pipe. Seal the now bare metal with grease. If the pipe is really bad, replace with a long braided hose from the underbody connector. This removes the metal and rubber hose in one job.