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I've been plagued by a persistent problem following my disc brake conversion last year and even the knowledgable people I've spoken too are not sure exactly what the problem might be.
When the car is cold and hasn't been running, the brakes tend to bind up during the first 30' - 60' minutes of driving. Cracking a bleeding valve on either of the calipers takes care of it (temporarily) but after driving a bit the problem recurrs. Strangely enough, once I've used the car for a while it seems to resolve, until parked again (say for an hour's lunch). The pedal gets increasingly hard and I can feel the car start to bog down. Before I ralized what it was, the first time I used it, the car refused to budge all together. Yesterday I pulled the car out of the garage after a 6 week rest and, while adjusting the tire pressure in the drive way, I noticed the brake lights come on(!) Sure enough, the pedal was hard and I had to bleed the brakes after driving all of 30 feet or so. The problem was pretty severe during the first 45 minutes of driving (had to pull over at least 6 or 7 times to crack the bleeder) and then eventually things improved.
The brake conversion was done by me as suggested on multiple websites (including this one) and used completely rebuilt (with stainless pistons) Girling 3-piston calipers, and entirely new parts for everything else from the master cylinder on back. (Yes, the rear system was replaced with stock parts including new wheel cylinders, brake drums, and hardware). All of the lines in the system were replaced. There is no booster and no pressure reduction valve. I'm using Castrol LMA DOT 4 fluid. When the problem first occured I took an older master cylinder I had and had it professionally rebuilt specifying that the one-way check valve NOT be installed. I'm sure that's the way it was done because the guy all but had me sign a release before he would do it that way. The invoice states 'per customer specs'. Needless to say, that did not at all resolve or improve the problem.
My thought is that the brake fluid tends to expand when it begins to warm up; the warmer it gets, the more it expands and somehow the fluid has nowhere to go.
I'm about to tear down the front end (tomorrow, I hope) for re-bushing and plan to fix the brakes in the process. For a different reason, I will replace the front lines again and plan to give the master an overhaul myself. If anyone has any ideas/suggestions before I put things back together again, I certainly will appreciate it. I did read a comment here a while back suggesting that there is a difference between the Lockheed and Wagner master cylinders but I thought for this application they were all made by Lockheed. Any thoughts on whether a sticking caliper could cause this?
I'll post this on the 122 board as well since the discs are more common there and my system essentially is a 122 system. Thank you all for you help.
Kai S
1967 Duett
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