My E Type somewhat recently acquired , seems to have a possible master cylinder sticking issue…car is a 68 ser 1.5… when coming to a stop at the lights, or anywhere you can feel the car come to an abrupt stop even when you do not have your foot on the brake peda.l…I have had the front callipers off and cleaned a couple of times… therefore I am thinkjng the M Cylinder is somehow sticking…
You are not alone. Did you do a search, there are many posts on it? Here is a recent:
Some causes:
- Not enough brake pedal free play.
- Swollen rubber hoses
- Sticking brake caliper pistons
- Small piston in the brake master cylinder sticking- seems to be one of the more popular causes
- Servo/booster, piston sticking
The threads should provide several troubleshooting techniques to narrow it down. Or, if you system has not been rebuilt is sometime, rebuild it all and get it behind you.
Tom
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Have you replaced any components recently, specifically the master or servo assembly?
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The place to begin is by checking the vacuum lines (there are quite a few of them). The most critical line is the one between the rear section of the servo and the reaction valve. Make sure all the clamps are tight. Maybe you get lucky and it’s a loose clamp.
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Get your patience dusted off…this may take some time and effort to work through.
Oh boy, is that ever an understatement!
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When I had this problem with my 68, I was given some excellent advice I’ll pass along. Disconnect the vacuum line coming off the back of the manifold (and plug it so you don’t have a vacuum leak that affects the running of the car). Now drive the car and see if you still have the brakes sticking. If you don’t, then you know your problem is on the vacuum side. If you do, then you know you have a mechanical issue.
And the car stops just fine with the vacuum assist disconnected! When I did this I discovered that I had a problem with the vacuum side, and I drove the car for a period of time without the assist.
My car had two problems. The PO had used fuel hose on the vacuum lines, which doesn’t have the strength to resist VACUUM as fuel lines are designed for handling pressure. Your local auto parts store will have the hose but be certain to tell them you need vacuum hose. If you just say brake, they will give you hose that resists brake fluid, but not designed for holding vacuum. You’ll need about 6 feet to replace all the vacuum lines. The PO also used hardware store style screw clamps. Vacuum lines should have solid clamps. Xks sells them for a reasonable price if I recall.
Yes, the car will stop just fine, just requires more pedal pressure. However, I continue to warn people that if they drive the car with the vacuum disconnected, they are disabling the one circuit of the dual brake system. That is, if the car loses the fluid in the MC circuit, then ALL brakes will be gone, not just one axle of brakes.
Tom
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This is the same advice that seems to pop up on every braking thread. Stopping the car without vacuum assist requires triple the foot pressure, and if the problem is actually the master, thre will be no brakes whatsoever. It’s not a safe approach. You can easily plumb a vacuum gauge into the system to test the pneumatic side safely. If you go to the other thread and review the solutions, you’re realize that knowing the problem goes away with vacuum disconnected doesn’t really help with diagnosis. In the latest example, sticky brakes were caused by a sloppy intermediate piston in the master, which has the effect of locking servo assist. In my case, the problem was a sticky servo slave, and the vacuum side seemed to be fine. So hydraulic problems can manifest as vacuum problems and vv. It’s a complex system, not easily diagnosed. Be patient, and don’t jump to conclusions.
Hi Tom, sounds great to me, where are u located…I am here in Vancouver BC…unfortunately young people here are putting us all at rish… as George Burns used to say " why is youth wasted on the young "//Best Wishes…to you and yours…
Hi Mike, I shall be carefull…where are u located… I am in Vancouver BC…Best Wishes…
Help me understand this. His brakes are sticking. He’s not experiencing brake failure or even longer stops than he feels are normal. So how could he find that all of a sudden he has “no brakes whatsoever”??
Am I missing something?
If he has normal, unassisted brakes they should easily stop the car. I’m not suggesting he take it out on the freeway, but a drive around the block or so should be easily accomplished. Like any time you start to move a car that you have just done something to the brakes, you take it slow, check the pedal and stopping capability and gradually expand, assuming all is going well.
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Robert, you are largely correct that this scenario is very unlikely. But. If the MC circuit lost all fluid, then the mechanical tripping of the reaction valve would come into play and the car would still stop okay. This is unlikely because for this to occur, the MC thus pedal would have to be going full stroke to to trip it and the driver would certainly know something is amiss. However, I feel people should be warned. I have seen worse. Next, again unlikey, the metal line from the MC to the servo could be crushed or blocked. In this case, the pedal would actually remain high, push the small piston, activate the reaction valve. and the car would stop. Disconnected the vacuum now would result in no brakes. Finally, I am concerned people would disconnect the vacuum, decide it operates okay and drive it long term that way. Now their backup system is gone, so if they lose a rubber hose or something they will have zero brakes on the highway.
Tom
An additional comment on the safety of disconnecting the vacuum. All modern brakes do have some type of dual circuit back up. Fortunately, that does not come into play often as brakes are very reliable. If we disconnect the vacuum on this system E Type, we lose the dual circuit. How dangerous is that? Well, most pre mid 1960’s and earlier cars never had a dual circuit. So my 1961 car has no dual circuit. So an EType without vacuum is no more dangerous than my 1961 car, which I drive without a second thought. But I still feel if I tell someone to disconnect for troubleshooting, they should know what they are doing. Especially in this case because on most cars, disconnecting the vacuum only removes power assist, not the dual circuit. And I have seen people post that misconception on the forum.
Tom
It’s not a misconception. You don’t lose the dual circuit. Without vacuum, the master pressurizes the slave, advancing the piston, which activates both circuits. (4.2 and V12 braking system, the 3.8 system is direct action to both masters if there’s a vacuum failure). Rather than an endless argument about this, here’s the direct quote from the 4.2 FSM:
The main danger in disconnecting the vacuum is the obvious: braking effort increases threefold. It’s ok if you’re doing it on the rack, but taking it on the road is a risk. It’s also bizarre that if you’re trying to determine whether the problem is hydraulic or pneumatic, you assume the problem is pneumatic and go for broke. But if it’s hydraulic, then you’re on the road with only faulty brake components. I wouldn’t want to be the armchair helping hand who sent someone out on the road with a deliberately disabled braking system as a test. I’ve been blindsided by the complexity of this system once too often to suggest that three won’t be a surprise result if you disconnect the vacuum and take it for a drive.
Logically, disconnecting vacuum really gives you no useful information. If the problem goes away, all you know is that vacuum is involved. That leaves you with the question of whether the fault is in the lines, a faulty booster or a faulty master. If it doesn’t go away, that means it’s not vacuum (or that it’s not ONLY vacuum.) Either in the slave or the master, and you still have to check the vacuum system. So no matter what the results of this dangerous experiment, you’re going to have to go through the problem by the numbers: Number one is check the hoses. Number two is R&R the reaction valve. Number three is R&R the master. And fin the worst case, R&R the slave.
Michael, what I stated is different than what you quoted. We are not referring to the same situation.
And I still believe what I stated is correct.
“Typical” dual circuit system. The single dual circuit master cylinder is combined with the vacuum booster. If one loses vacuum, yes, pedal effort does increase. But the mechanical pedal still pushes (through the in active booster) the first MC piston which pushes fluid to first axle, and the fluid also pushes the second piston which pushes fluid to the second axle. Now, if in the condition of someone disconnecting the vacuum, one now loses the first axle fluid. The pedal still pushes (without vacuum) the MC first piston. There is no fluid there due to loss. That first piston then mechanically pushes the second piston which pushes fluid to the second axle to brake. Now if the second axle had lost fluid, then the MC would push the first piston, which would push fluid to the second piston. the second piston would bottom out, then the fluid trapped between the 1st and 2nd pistons would go to the first axle for braking. One of the dual circuits would always work regardless of a single fluid loss anywhere
The Jag system. If the fluid loss was the lines from the slave to either the first or second axle, then the scenario above would be accurate.
But- if one disconnected or lost the vacuum, such as at point Y, one would have brakes without boost through the MC fluid to the slave, no vacuum. But if without vacuum you were now driving the car and had a failure as X, I see know way anything is going to move the slave to have brakes. That is why I say this is NOT a typical dual circuit system. It is different than most in that it is dual from the MC to the slave, and then dual from the slave to the two axles.
Correct me where I am misreading.
Tom
Ok, sure. If you lose vacuum AND hydraulics, you have no foot brakes. Which sounds like a good reason not to experiment with disconnecting vacuum. Thank goodness for the hand brake.
Tom, while I agree all of this is possible, it seems rather unlikely for a short, cautious, trip to try to isolate the problem between vacuum and mechanical. Again the op has a sticking brake. He’s having no issues with stopping. It’s getting going again that’s the problem. I know I had the exact situation, and blocked the vacuum line with a pen, drove down the block and no more sticking brakes! I knew instantly that it was a vacuum problem. It worked for me. And nobody died.
If you look at this photo you can see the black pen shoved in the vacuum line coming off the rear of the manifold. 
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Robert, yes, I agree that a problem is unlikely. If you have been following my threads, I have stated so. If I suggest something that I feel may have a possible unknown concern, I attempt to let people know.
Tom
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Hi Bob, what size vacum hose is it…prob 5/16 I guess.? BTW not sure why these e mails say they are from Jerry Mouton…Jerry passed away several years back shortly after I met him in Victoria BC… Just before I was kicked out of the old boys and bitches club JCNA,… now being run by westpoint military… I do not think these ex military should be allowed to govern civvy clubs… Regards Art…
Hi Art. Sorry but I don’t recall. I’m almost certain the clamps are 5/8”. And vacuum hose with a 5/8” OD probably only comes with one ID, as the wall thickness has to be able to hold up against the vacuum.
That’s what my old hose was doing, that is, collapsing. My guess when new it might have been fine, but the sharp bend off the booster, combined with the pull of the vacuum, caused the hose to pinch.
The ID of the hose is 3/8". Gates vacuum hose would have a 5/8" OD.