Locked up brakes

[awiedie]

Maybe someone has some ideas about a problem I have experienced with my brakes, as recently as yesterday. I started the car in the garage, letting it sit while on the choke, and when that kicked off let it idle to see how that went. Seemed to be better than it has been and so decided to back the car out of the garage. At first I thought I had the emergency brake on, but it was not. Finally decided the brakes were engaged and it was difficult to move the car forward or reverse while in gear. I could not roll it by hand with the engine shut off. This morning it would roll ok.

This happened to me once before after a run a couple of years ago and I seem to remember that at that time, the brake lights were on even with the ignition off, until several hours later when they went out. It seems like something is maintaining pressure in the system even when the brake pedal is not depressed. Could it be the servo? which I replaced with a new one when I rebuilt the engine compartment a number of years ago. Any thoughts appreciated. As an added note, the brake pedal is exceedingly hard with practically no give to it.

[cass3958]

Could be a couple of things. First do you have the anti creep brake system on your car? With the DG Auto system if you stop the car on the brakes the brakes remain on until you depress the accelerator with the anti creep system. I don't have it on my car being a 1968 with the BW35 box but someone on here might know more about it. If the anti creep if coming on you might need a bit more accelerator to release it rather than just creeping out of the garage on tickover. Try holding the car on the hand brake which uses a cable rather than hydraulic fluid and with the car in reverse gear, rev the engine up slightly then release the hand brake. Don't use the foot brake to hold the car as this will defeat the object until the car is rolling. You could even jack the back of the car off the ground and try this but more revs might free the anti creep.

Second if the brake pedal is solid you might have a seized master cylinder brake piston which over time releases and allows the pedal to go back up and the pressure to release.
It could be a seized brake caliper but this would only effect one wheel so jack the car up and check if it is one or all the brakes locking on. If one it will be a brake cylinder if all then forget this suggestion and check the top two.

As you have worked on or replaced the servo check that the piston in the servo has not seized. Check you have a decent vacuum pressure to the servo.

Lastly when was the last time you replaced your brake fluid. Glyn will tell you that brake fluid can become water logged that makes the brakes spongy but also dirty brake fluid can silt up and you might have a blockage which is stopping the brake fluid from releasing its pressure. If this was the case and only the front or back brakes were locking we can tell where the blockage might be ie in the front or rear brake pipe system. If all four are locking then the blockage will be between the master cylinder and the servo.

[John Quilter]

Very good suggestions by Cass. I would also add when testing the car when it is on stands see if the LH or RH front caliper is causing the drag. If just one, open the bleed screw on that one and see if it releases. If yes, this could be an indication that the rubber hose from the metal line to the caliper is perished internally and forming a one way valve preventing fluid from returning to the master cylinder. When this happens usually within few minutes it will self release. A similar check can be made with the rear calipers but these are fed by one hose only.

[Glyn Ruck]

Alan,
Robb has covered the usual suspects & your car is dry stored so there is no chance of pads rusting onto discs/rotors.

My guess is the anti creep system. You have been messing with carbs and the accelerator switch might not be releasing the rear brakes when you accelerate. Check that the throttle switch is operating properly. Otherwise it could be the pressure control switch in the transmission or the main pressure solenoid itself. Most likely switches. The circuit does not hold pressure forever which is why yours finally released. The wiring is all messed up on the underbody pics but they are all I have of the main pressure hold solenoid.

Hard brake pedal after starting can also mean servo trouble. Pedal should soften a little after you start the car & build some vacuum.

Good luck.

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[awiedie]

Thanks for all the thoughtful advice - it looks like I have lots more investigation to do. One of the first things I need to do is get the car up on jackstands to see if the braking affects both front and rear. If rear only, the anticreep system is suspect, if both then could be in the new servo applying pressure to the whole system. If I understand the system correctly, with the key off, the anticreep should be inoperable and the wheels would turn - that's another test.

I just keep thinking that I drove this car for almost 40 years without this happening, and after putting in a new servo I have the problem. I don't think I even had the anti creep switch at the carbs adjusted for contact all those years which would mean the system was never working at all. Now I have the switch adjusted properly.

Robb, brake fluid is 4 or 5 years old. I'll check pressure at the servo also.

[Glyn Ruck]

Yes ~ Make sure which wheels are braking. I actually intended putting that in the post & then a buddy phoned from Dallas & we've been chatting since then. Ignition off = no anticreep as long as no one has fiddled with the wiring.

Brake fluid is hygroscopic. Change every 2 to 3 years.

As I say a good test for an operating servo is to put your foot on the brake pedal then start the car & you should feel the pedal soften. Don't use any exotic brake fluids. DOT 4 or DOT 4+ is fine. Servo elastomers are seldom compatible with them unless specifically stated.

Good luck

[NigelW]

I've seen this before with Jaguars, I was with my brother attending a Jaguar meeting at Beaulieu in his Mk2 and his brakes locked up when we went to leave the field. We had to crack open one of the brake pipes to release the fluid pressure, we got as far as the exit to the New Forest when they locked up again not wanting to risk going any further we had the car recovered back home. On further examination the problem turned out to be the master cylinder. I had the same problem with a friend and his Mk2 - same fix, the seals in the MC had swollen and the piston was sticking, the Mk2 and the S Type have essentially the same design of brakes.

[Glyn Ruck]

Interesting!

I had MC trouble with new seals not protruding sufficiently to close the inlet port. Had to shave the tip of the piston slightly. Sometimes had brakes & sometimes not.

[awiedie]

Thanks, Nigel, I am definitely going to look into this possibility. As part of my engine compartment rebuild a few years ago, I had the master cylinder re-sleeved with brass and of course put in new seals. Doesn't mean they might not be defective. On a related note, similar to Glyn's comment, I had a problem with clutch operation on my 66 E type in that I had no pedal and then I did. The system was filled with silicone. When I dismantled the master cylinder, I discovered that the seals had just about dissolved, particularly the little no-return one on the end of the piston, so I was just pushing fluid back into the reservoir rather than out into the lines and therefore no clutch action. New seals and DOT 4 and no more problems. Oddly enough, the brake system still has the silicone in it (don't ask how many years) and has given me no problem.

[John Quilter]

I've run silicone fluid in my clutch and brakes for decades with no issues on my 3.8S. I also run it in four other vintage British cars with good results. Two Morris Minors, one MGTD, and one Austin America.