Variable brake pedal

[USAndretti42]

It's a long time since I drove a car with a vacuum servo so I cannot remember how they feel. Getting old. The memory's going.

Usually when I brake, the pedal has only slight travel and is firm. The car has braided brake lines, by the way. Often, when I am sat with my foot on the pedal, say at some lights, the pedal drops sligtly beneath my foot, which, I think, is the servo acting. I don't press harder for this to happen. It just does it while I am stationary, not knowing where to look in case someone is staring at the car. Yesterday, though, it dropped a lot lower than normal after first dropping to the normal point. It returned to normal (firm pedal, little travel) a few pedal presses later although not by pumping the pedal, just normal brake applications as I drove. It did this twice. Today it was OK all day.

Any ideas?

[DanR]

Might need renewal of the seals. Being a tandem system there could be a wear and a leak passing the internal seal from front chamber to the rear.

[chris]

Hi, when was the brake fluid last changed???

[molemot]

Sounds like the master cylinder, Trevor...you may get away with changing the seals, but if the fluid has been in there for some time, you may have corrosion where the water collects in the bottom of the bore, fluid being hygroscopic. Happened to my clutch master cylinder whilst I was doing my protracted respray!

[USAndretti42]

Changed the fluid and the seals 2 years or so ago. What is odd is how intermittent the thing is. The movement from the first position to the second has been going on for ages. The movement to the third is a new thing. It's not a gradual sinking of the pedal but a sudden drop to a new position.

Only did it the one day, which is odd.

[peteyg]

Air in the system? Could be the obvious things.

[WayneB]

Trev, I suspect air in your braking system over a servo problem is causing this wierd intermittent increase in pedal travel.

I would re-bleed the system first and try it before getting into replacing or rebuilding the master cylinder(if the bleed job doesnt solve your prob)

[molemot]

When I overhauled my front calipers, I found I had a lot of pedal travel... this was due to the pistons retracting into their cylinders, as they were no longer all rusty, grotty and sticking! I fitted a brake pressure valve to the front brake lines, this keeps a few pounds of pressure in the lines to the calipers and stops the pistons retracting too far; gave me a really solid pedal. The valve is sourced from the USA where it's used by hot rod builders, among others, where the master cylinder is below the level of the calipers and it stops the fluid draining back. Probably not too relevant to your intermittent troubles, but could be that your pistons are migrating back into their cylinders occasioanally giving you the pedal travel.

[USAndretti42]

The symptoms got worse and worse until the pedal was sinking quite a way every time. I tried bleeding the brakes and, as expected, there was hardly any air in the system and it made no difference.

Like DanR and MoleMot I concluded that the seals for one circuit were leaking in the master cylinder so that the front or the rear (never did decide which) were ineffective. This was a bit disappointing as I changed the seals when I first had the car 3 years ago. If the seals were leaking, there must be a fault in the cylinder bore. The master cylinder already had a crack in one of the mounting flanges so it was time for replacement.

New S3 brake master cylinders are to be found only on the backs of unicorns, surrounded by hens' teeth so I rang up LotusBits. They pointed out that the servo was incompatible between the S2 and S3. The S2 servo is sealed to the brake cylinder while the S3 has a seal in the servo and a breather port in the mounting face of the master cylinder. So I got a servo as well.

Despite the best efforts of FedEx in the USA (when they picked it up from customs, they put a new address label on the package but didn't have the correct address on it), I got the parts a few days later. Of course, it's not a straight swap. The S3 servo is bigger than that of the S2 so yo have to grind away the fibreglass to get it to fit. Another annoying thing is that the S3 master cylinder pipe connections are both 3/8" while the front circuit connection onto the S2 cylinder is 7/16" meaning I had to get a new piece of brake pipe. No big deal just a delay.

The non-return valve in the servo is smaller on the new piece and in different position needing a new hose from the front of the chassis. Again, no big deal.

Anyway, all the new parts are fitted and the brakes bled and all is well. Brakes are nice and firm and the parts cost only