Brake quandry
 

My 68 (series 1 brakes) has developed excessive brake pedal travel. The brakes have been bled and most all components are about 2 years old. The pedal travels almost half way to the floor, rather disconcerting. Brake performance seems otherwise okay. Pumping the brakes has only a small effect. Removing the front brake line from the brake servo and plugging the hole results in a solid pedal with about 1/2 inch travel. The rear brakes appear to work okay like this. I assume this means there is not air trapped somewhere and the brake components excluding the front calipers are good.

I reattach the brake line but disconnect both front brake pistons and the pedal is still good. Connect and bleed one front brake (either 1) and excessive movement returns to about half the original. Reconnect the second brake brings back the original situation. I bleed the brakes by pressurizing one fluid reservoir; this has worked well in the past. Keeping slight air pressure applied, brake pedal motion is very small and all brakes seem okay, meaning they grab and release. Any thoughts anyone?

Master and/or slave cylinders perhaps?

old flex lines can cause some of the issues you described.

Thanks for the input, guys. When I disconnect and plug the brake lines at the front calipers, the pedal is fine. I think that points to the front brakes themselves but it is hard to understand why connecting either one of them causes the problem to return. I will keep plugging. At least it is not driving season,

Question. Are you losing brake fluid at all?

Late to respond I know, been trying to finish some painting before it is too cold. No, I have not been adding brake fluid until just now when both front wheel cylinders (I think the inside ones) started serious leaking. This is a new development which is probably not my original complaint. Painting is done so back to the investigation.

Bleed farthest cylinder first ( right rear ), then Left rear, right front, and last left front. Unless RHD would start left rear. Longest lines would then be cleared of air first.

For anyone following along, the problem was the front brake piston adjustment mechanism. The pistons were retracting about .05" when the pedal lifted. Spec is .008 to .01. That extra gap made the pedal travel excessive. All 4 cylinders were replaced together a few years ago (but not many miles) and with some pad wear the gap became apparent. Nothing a few dollars won't cure. At least I have my sanity back.

I deduct that you have the old-type wheel cylinders which had a pad withdrawal mechanism inside: -

(web pages by SNG Barratt)
https://www.sngbarratt.com/us/#!/Eng...FRONT%20BRAKES
the Series 2 brakes had the more modern caliper with pad withdrawal acheived by the hydraulic seal itself distorting slightly when the brakes are applied and springing back when released.
https://www.sngbarratt.com/us/#!/Eng...FRONT%20BRAKES

Why not modernise your brakes or is it rather too expensive ?

Fraser, you are right with your assumptions, cost is the issue. Too many other places need the cash. Also as the car and I age, we do not drive as aggressively.

But if you do find that pot of gold at the end of the rainbow, buy Zeus or Coopercraft, but make sure you buy both front and back. Or if cash still isn't forthcoming, did you know that Volvo 240 discs and calipers will fit the front with just a slight modification? Cheap at the breaker's yard too!

These original-type wheel cylinders are not cheap are they !

https://www.sngbarratt.com/us/#!/Eng...5-d0a204fba30b

And four are needed for the front.

When Dunlop were developing the disc brake for cars, they puzzled over how to withdraw the pads after a brake application. They came up with the solution found on early disc-braked Jaguars. The pin that sticks out the back of the cylinder is secured by a distortable little washer clamped to the back of the cylinder. When the brakes are applied, this distorts as the piston moves under hydraulinc pressure and then relaxes back when the brakes are released. Clever, eh !! Inside the piston where the pin runs is a holding clutch that allows movement under hydraulic pressure when the travel of the distortable washer is taken up, and thus caters for pad wear. All rather expensive to make, and some clever guy somewhere in Girling thought up a cheaper and better design by eliminating the pad withdrawal mechanism. This design, now in universal use by all auto brake manufacturers, has the hydraulic seal mounted in the cylinder and the piston is chrome plated, and pushed into the cylinder past the hydraullc seal. Under hydraulic pressure, the piston moves outwards, first by the seal distorting and then, if there is still clearance, past the seal until the pad contacts the disc. When the brakes are released, the hydraulic pressure ceases and the seal relaxes thus withdrawing the piston.