I was recently driving my 1971 Series 2 E-type on a warm day in traffic, and noticed several instances of the brakes not fully releasing when I took my foot off of the pedal. Each time this happened, I noted that there was increased freeplay in the brake pedal, indicating that the master cylinder piston rod was not returning to its extended (released) position as it normally would under spring pressure.
After reading several posts on this forum, I pursued the theory that the problem was that the master cylinder piston rod was binding in the plastic bearing, due to inadequate clearances and effects of thermal expansion. It should be noted that the master cylinder on my car was an aftermarket unit supplied by S.N.G. Barratt. This unit had a black plastic I.D. strap wrapped around the cylinder body.
I decided to do an experiment to confirm the above theory. I removed the binding master cylinder from my car. I happened to have an original master cylinder that was removed from another E-type. I replaced the seals in that unit. I placed both cylinders in a pan and placed them in an oven at 190° F for 2 hours. I removed them, and operated each one by hand to check for binding. The original OEM unit operated normally with no binding. The aftermarket unit stuck in the fully retracted position. After sitting on the bench at room temperature for about 15 minutes, the aftermarket unit released and banged into the extended (released) position.
Upon disassembly of the aftermarket unit at room temperature, I performed a dimensional inspection of the piston rod, bearing, and cylinder bore. I found that the bearing was installed in the cylinder bore with a .006 inch interference fit. The clearance between the bearing I.D. and the rod O.D. (disassembled) was .007 inch. It is clear that installing the plastic bearing into a cast iron cylinder with such a large amount of interference fit would cause the I.D. of the bearing to be reduced by several thousandths of an inch. I then removed material from the O.D. of the plastic bearing until I achieved an O.D. slightly less than that of the cylinder bore. I reassembled the unit and repeated the oven test at 190° F for 2 hours. I again operated the unit to check for binding. No binding occurred, and the unit operated freely.
This would seem to confirm the theory that the extreme interference fit between the cylinder bore and the plastic bearing, in conjunction with the effects of thermal expansion, caused binding between the piston rod and bearing resulting in failure of the brakes to release. I can see no reason for the interference fit between the cylinder bore and the bearing. Additionally, I noted that when disassembling OEM unit, the plastic bearing was not tight in the bore.
Measured dimensions:
• Cylinder bore for bearing 1.170
• Bearing O.D., original 1.176
• Bearing O.D., modified 1.165
• Bearing I.D., unrestrained .715
• Piston rod O.D. .705
The unit that had the binding problem has the following identifying features:
• Black plastic strap, similar to a zip tie, wrapped around the cylinder body
• Strap indicating the following characters: G26F 4212 556 L5L
• See the photo of the box that the unit came in
I would recommend that this aftermarket unit be avoided for future purchase. It may be pretty hard to differentiate between the suspect unit and others. One approach is to have your original cylinder re-sleeved at Apple Hydraulics or White Post Restorations, and install new seals.
Excellent work Mark! Many of us, including myself, have had the same symptoms and reached the same conclusions as yourself, albeit without the rigorous comparative measurement analysis that you’ve done. I have one such Master Cylinder stashed away somewhere in the garage that I removed and replaced about a year ago. The replacement, also from SNG Barratt, has performed fine since then, having covered 3000 miles or so. I will take a look at it to see if it has the same strap and markings as the culprit in your case. I will also dig out the faulty item and see if it has the same markings. Have you contacted SNGB with details of your findings? I would hope that they would welcome the information and use it to address the problem.
Thanks, David. I have not contacted SNGB yet, but I think I will. They should be aware that they are buying parts from amateurs.
920E is essentially the OEM of the original parts. AP Hydraulics, the well known brake manufacturer, acquired the Lockheed brake business.
Then in modern times, AP spun the racing and OEM business off into separate entities. Liberty Group acquired the AP OEM business and forked part of the business into 920E.
This is why SNG calls the above part OEM - it is the OEM in its current incarnation.
Dave