420 project update with braking question

It’s been awhile since I posted anything on my 420 project. Last time I was swapping in a 4 speed overdrive transmission in place of a 700r4 conversion. A kind member on this site did a trade. The overdrive accumulator piston had exploded which is why it didn’t bench check. With some modification of a readily available piston I got it repaired. I didn’t do anything but reseal both ends and top cover of the transmission. I wound up rebuilding the engine when I discovered head gasket weeping on the intake side from luckily minor corrosion.

Today I got it out for the first road test. Not finished but enough to get a test run in. The transmission worked well. A little growl in first for a split second and it shifts well and synchros seems to work well in all gears. I don’t have the electrics connected to the overdrive yet, so haven’t road tested that yet.

On the down side my brakes are giving me issues. The servo assembly isn’t releasing the pressure to the rear brakes. If I crack the line pressure releases and it rolls easily. If I apply the brakes several times they drag heavily. Does anyone have ideas on this problem? I hope I don’t have to fiddle with the servo a third time and have to bleed the rear brakes. It was pure he’ll last time.

I will assume the rear rubber line is new (you can crack the line there and see if it releases the rears

dont know what master cyl you have on your 420, but an improper fit of the master cylinder seals that control the rear calipers was the cause of issues with my 420G

Luckily, as I had it re-sleeved, I was able to get a top brake expert to sort it out on the bench
( he told me the correct part num seal kit I supplied had rear seals that were too long)

Does the 420 have the hill hold relay in the rear brake line?

Brake system is a new last fall. Hoses and lines as well.
I think it’s a sticking piston or seal for the rear brake circuit. I tapped the servo with a dead blow and for the moment it seems fine. I hope it will behave. This car has sat since October last year.

A new discovery. I installed a electric fan and had not installed the relay kit. I did that yesterday and ran the car to test it out. I applied and held the brakes and the car moved fine when released. After running a bit longer and not applying any brake it did it again the rear brakes were grabbing enough I could barely move the car by hand. I think heat from the exhaust was warming the rear calipers and caused the fluid to expand. I wonder if the check valve in the servo for the rear brakes is to blame?

I had the same problem with the Mk10 when I briefly had it back on the road after reworking the brakes. Is the booster in-line like the Mk10? If so, make sure you don’t need a spacer between the master and booster. On mine, the booster was activating with a neutral pedal after things warmed up. I also don’t like the internal anti-retraction pins in the calipers as they come from the factory. I delete them and put a Wilwood residual pressure valve in the line, 2 lb for discs.
Check valves in the servo are commonly sited as a problem. I think you could probably delete that if you use in-line RPV, but I haven’t done that.
Great job on the transmission - I’m glad it worked out for you.

My servo is in line, but dual circuit like late E-types. No spacer called for. I thought about deleting or opening the bleed hole on the check valve for the rear brake circuit.

No spacer was called for on my application either (dual circuit series 2 XJ6 into Mk10), but it was needed. You can easily tell if it’s the problem by loosening the two nuts connecting the booster to the master and see if the rod touches the booster input before the bolts snug up. Mine only took 1/8" shim, but that small amount created a major problem at the wheels. Your issue may be different, though.
It has been written here that some of the remanufactured boosters have a check valve problem. I’m not sure how either of those things would affect only the rear brakes, though.

My servo is the remote style in left wheel well. Master cylinder is fully released.

Ok. I was confused by the description of it being “in line”. Not very fun to access those remote boosters - I remember my mk2 was a bear, and I installed it before I put the engine in. But, easier to get to that than the rear calipers.