Spongy Brakes

[Trevor John]

When Dick Lovett last serviced my car (a 1983 SC Cabriolet) they suggested that the brake fluid should be changed. Afterwards they tried to bleed the brakes a number of times but they remained spongy, so they changed the master cylinder. The brakes still seem spongy to me ie when you push the pedal hard, it seems to go down until it hits a stop. On the road they are poor compared with their previous performance. Could there be some other fault that has mysteriously developed when the fluid was changed eg a faulty hose? Or are they just bad at bleeding brakes? Any suggestions?

 

[bones]

If the brakes are no better than when you took them in I suggest you complain loudly as you probably payed through the nose for them to fit the new Master cylinder which han't cured the problem. However at least it is a worthwhile safety feature on a 20 year old!
If you have new fluid, healthy pads and calipers then it is most likely a brake bleed problem or the old hoses that attach the hard lines to the calipers are expanding under pressure. With an '83 year car I suggest you invest in some stainless hoses and rebleed the system a number of times till it feels right with ATE 'blue' brake fluid. It is not always easy to get a system bled even with pressure bleeders, sometimes it takes a few goes.

 

[AirCooledExcellence]

Bones has some good suggestions here. I had the same problems with my car and went through the process of replacement of master cylinder, re-building the calipers etc..

What fixed the problem in the end was ensuring that the pads were a tight fit in the caliper. Take them out one at a time put some pressure on the brake pedal so the piston moves out slightly then try and get the pad back in. If the piston is too far out push it back a tiny amount at a time, until you can knock the pads in with a small hammer.

Other than that do what Bones suggests then bleed, bleed and bleed some more!

Good Luck!

Tim.

 

[oilyrag]

It's also a good idea to bleed well, leave overnight and bleed again the following day, I did this when I changed to silicone fluid which loads of people say gives a spongey pedal feel but after this treatment using a pressure bleeder my pedal is as firm as the next mans, if you get my drift.

I put this down to what looked like micro bubbles of air in the bled fluid the first time I did it, which when left to accumulate in some high point in the system are easier to bleed the second time around ....at least that's the only logical explanation I could come up with; my system had been completely stripped & overhauled before changing to silicone, which has the added advantage of not absorbing moisture so does not need regular changes & prevents corrosion in the system.....I think this latter point important in a car that stands idle for some time in the year. [8|]

 

[bones]

That' s an interesting theory about the micro bubbles oily, I've thought that too. The system is quite large too so maybe bubbles can 'attach' themselves to the inside of the pipes and only release later after the car has been driven and the bubbles have been shaken free, hence a bleed the next day would cure the problem.

 

[Trevor John]

Thanks for the helpful advice. I do not have a pressure bleeder (except for the wife, and she's too expensive!) but I see that Frosts do one that works off the air pressure in a tyre. Does anyone know if this works OK? If not, any suggestions where I should buy one?

 

[oilyrag]

Yes Trevor this type work well, I think mine is a Gunsons one from Halfords.
However........there is a word of caution when using one of these on an early 911.........

There is an overflow/vent pipe attached to the top of the brake fluid reservoir which links into the petrol tank overflow pipe in the boot....this runs to the front of the boot and exits through the front panel (from underneath this is behind the valance)....anyway it is ESSENTIAL to detach the 1/8" overflow pipe at the reservoir end & plug it off (I use a suitably sized self-tapping screw with a pan head ie not countersunk).....this avoids all your precious fluid dumping on the floor when pressure is applied to the reservoir!

Another small tip is that these bleeders leave the reservoir completely full.....so care is needed when detaching the bleeder cap, I then remove a small portion of the fluid to leave it at the max mark using an old "Toilet Duck" plastic bottle to suck it out. [8|]