Locked brakes

[awiedie]

I recently did quite a bit of work in the engine bay on my 1964 3.8S, including a complete brake overhaul. I rebuilt the master cylinder, installed a new brake booster, replaced brake and vacuum lines and installed a new brake switch. At the end of a short test drive, I left the car running in the driveway while I opened the garage door. When I went to drive in, it felt like the brakes were on, which, in fact, they were - without my foot on the pedal. When I did put my foot on the pedal, there was NO movement at all and it felt like I was standing on a giant rock. I managed to get the car into the garage but the brakes were still locked up and the brake lights were on and continued to remain on for some time after shutting off the car. Eventually the brakes freed up and the lights went off.

I'm thinking this is some problem with the booster but would appreciate other comments. I did not replace the valve connector on the vacuum tank, but this system has worked fine in the past. Help!

Alan Wiedie

[Jose]

I used to have that problem with my 1992 Ford Aerostar minivan when the brakes became wet and I engaged the handbrake for the night. If the temperature was cold, the rear drum brakes would freeze in place, so I learned to not engage the handbrake in cold weather.

it does sound to me like a vacuum issue. Don't ask me why, I don't know why.

[Jose]

here's another opinion from more knowledgeable people:

https://www.popularmechanics.com/cars/h ... -15146608/

[johngosnell]

Just a thought, but as you have rebuilt the master cylinder is it possible that the piston is sticking and not allowing the master cylinder to go return thus keeping the brakes on. if you crack the brake line and allow brake fluid out the brake should release, Just a suggestion .

Regards John

1B1257BW
1972 Rover P6

[cass3958]

Interesting Alan. I am guessing as I have never come across this before. The brakes obviously work on fluid pressure where the master cylinder tries to compress the fluid which in turn compresses the slave cylinders which deploy the brakes. The brake lights work off a pressure switch in the brake line so the fact the brake lights remained on means there was no release of the pressure in the brake lines.
The pressure is normally released simply by lifting off the brake pedal and the fluid will return to its normal level in the slave cylinders and the brakes release. The Booster or brake servo is just a chamber that with the use of the vacuum from the engine provides extra pressure on the fluid so you do not need to push the pedal so hard. It does not interfere with the flow of the fluid as it is more of an aide to the working of the pedal pressure. But if the vacuum pressure is not released when you stop pushing on the pedal then the brakes will remain on. The fact that the brakes eased off after the engine was killed suggests that the vacuum had not released in the servo/booster. There should be a check valve or non return valve somewhere in line from the engine to the servo which I think is actually on the side of the servo in the Jaguar. I remember when I built my Lotus Seven it failed a test because I had not fitted a non return valve or check valve in line which allowed a certain amount of vacuum to be retained in the servo in case the engine failed. My feeling is that you either have a faulty check valve or it has been installed back to front and is retaining too much pressure on the pedal.
This is a link to an article on the internet which explains how the servo or booster works.
https://auto.howstuffworks.com/auto-par ... brake1.htm

[NigelW]

I have come across this problem before many years ago with my brothers Mk2 Jag. We were coming home from a jag meeting about a hundred miles away and when we stopped for about ten minutes the car wouldn't move due to locked on brakes. We got it going again by bleeding off some brake fluid. The problem turned out to be the servo and was duly replaced. We didn't pull the servo apart to find out the exact cause as it was replaced on an exchange basis.

[awiedie]

For some reason last night, I started to think about the locked brakes problem on the S Type and thought maybe I would look into a new servo as I remembered thinking that was a possible solution. I Googled Jaguar 3.8S brake servo and one of the search results was something titled "Locked Brakes" International S Type Register. Much to my surprise, this turned out to be my post from March, 2018! How time flies. I feel like I've been on Mars for the last six years because none of the responses to my post are familiar at all. I certainly don't remember getting email notification of any replies, several of which are very helpful and I certainly appreciate them. Thanks to Jose, Rob, John and Nigel.

Anyway, I'm hoping to get back to the car this winter. I still don't know if the servo is the problem or not and certainly don't want to spend hundreds and hundreds of dollars on a new one or even a rebuild kit and then find it was something else. Is it a bad vacuum line, one-way valve on the vacuum tank, or something internal to the servo. One new bit of information is that a couple of years ago, I had the car out idling in the driveway and noticed that the brake lights were on. Sure enough, the brakes were locked up. I went and got a hammer, and with the butt end of the wooden handle smacked the edge to the hexagonal valve on the servo. The brakes immediately released. Weird huh? Anyway, it'll get figured out some day.

[John Quilter]

Might not be your issue but if the master cylinder is like many there is a return port that allows fluid to return after the brakes are applied. There is a critical adjustment as I recall on the linkage/pushrod the allows this port to be open when the pedal is not depressed. Since you rebuilt the existing master cylinder could you have disturbed the free play at the pushrod?

[Glyn Ruck]

Alan, while boosters and the air/vacuum control valve are frequently the cause of this problem your car has a BW DG250 Automatic transmission. and is thus fitted with the Anti Creep system that can cause locking of the rear brakes only if not in good order or the throttle switch is not adjusted correctly. The fact that giving the vacuum valve assembly on the booster a clout released the brakes & the brake lights went out would lead one in the booster direction. Just don't forget about the anti creep system as a possible cause.

Anti Creep System.JPG (100.54 KiB) Viewed 1254 times

Anti Creep System 1.JPG (64.46 KiB) Viewed 1254 times

Anti Creep System 2.JPG (60.58 KiB) Viewed 1254 times

[awiedie]

John, thanks for your comments. Having rebuilt the brake system quite a few years ago, I had to go back to the manual to refresh my memory on how things work. There are only the two ports on the master cylinder - from the reservoir and to the system with a seal on the end of a valve to prevent fluid from going back to the reservoir during braking. As an aside, I had this problem on my E Type one time where the small rubber cup seal on the end of the clutch master cylinder piston had deteriorated and every time I pushed on the pedal, all I was doing was pushing most of the fluid back into the reservoir, hence, gear changes were an adventure. I was using silicone fluid and switched to Dot fluid in that system but am still running silicone in the brake system - since 1990. Anyway, there is no adjustment on the S Type brake master cylinder push rod as it is of a fixed length with a clevis and pin attaching it to the brake pedal. I think I'm leaning toward the booster as the culprit.

Glyn, thank you also for the comments and diagrams. First, as to the anti-creep system, I could never get that adjustment correct at the carb linkage so I just disconnected the bullet connector at the switch and therefore have no anti-creep. The booster, of course, is a whole other can of worms. During the course of trying to figure this out, I studied the diagrams and text in the workshop manual as to how the system worked. Seemed pretty complex to me, especially that air valve with all the seals and springs! A rebuild kit is available for the booster but doesn't contain much. Maybe a new unit is in order but they seem to be hard to come by and are quite expensive.