Brakes, just no power brakes

(randy olson) #1

My 1969 E-Type 2+2 has a braking issue. More specifically, the brake booster isn’t doing it’s thing…assisting with the stopping of the car. Here’s a few observations:
The car sat all winter, and hasn’t been driven in six months (except for a 3 mile test drive). When I put it away last fall the brakes had no booster, but the car stopped when I braked. It still does.
I had to add about a quarter cup of brake fluid to the fluid reservoir after noticing the red light flashing on the dashboard when I test drove it.
I tested the reaction valve and it seems to work. I started the engine. I took the hose off the plastic cap and there was good vacuum. When the brake pedal was pressed, the vacuum stopped.
I also put a “dipstick” into the vacuum canister to see if it was full of brake fluid. There may have been a quarter inch of fluid on the dipstick, but there wasn’t much.
The master cylinder and servo were rebuilt by me in 2006. I probably rebuilt the slave cylinder, but not sure. The master cylinder bore was pitted, but I didn’t consider it bad enough to replace it…in retrospect I probably should have.
In any case, I’m looking for some direction as to what to check next. Or, do I just buy a new master cylinder with the reaction valve? Do I need to pull out the servo and see if it contains brake fluid?
I’m puzzled by the fact that there’s vacuum on both sides of the servo diaphragm, but there isn’t any brake assist.
I’m hoping there are some more tests I can do before tearing into the master cylinder, servo, and slave cylinder. Please let me know if you’ve got any ideas.
Thanks much,
Randy Olson
1969 2+2, British Racing Green/Tan

(Geo Hahn 1969 Series 2 OTS) #2

I think you have to wonder where all that fluid went, especially if there was no evidence of a leak.

I also did the ‘dipstick’ check of the vacuum canister and found it dry, but when I removed the booster this was inside:

Those brakes worked fine, but fluid was disappearing to somewhere.

BTW, the 2 layers of fluid are because I switched to DOT5 7 years ago but the problem obviously predated that.

(randy olson) #3

George,
Was the brake fluid in the servo due to a failure of the master cylinder, slave cylinder, or something else?

(Geo Hahn 1969 Series 2 OTS) #4

Possibly the master at fault, possibly both. I am either…

a. lazy
b. cautious
c. thorough
d. or foolish

…but I simply replace both the master & servo and now all is well.

The problem (whatever it was) never affected braking, I had just observed that I was having to add fluid to the rear reservoir from time to time with no apparent leak.

(Ray Livingston) #5

Think about that for a second… How could a problem with the master cylinder possibly result in fluid accumulating in the vacuum booster?

That can ONLY occur due to a failed rear seal on the servo cylinder.

Regards,
Ray L.

(Geo Hahn 1969 Series 2 OTS) #6

Isn’t it possible for a failure in the seal at the front of the master piston to allow fluid into the reaction valve which then gets into the booster when the engine is shut down?

(Andrew) #8


A leak at A will let fluid into the booster E. A leak at the reaction valve piston seal will let fluid into the vacuum line which could go into the booster E or even F if the reaction valve diaphragm is holed.

To the OP. It’s worth getting your head around this diagram. Once it makes sense you can take a logical approach to your lack of power assist.

Essentially

no vacuum P
vacuum valve is broken Q
reaction valve is broken RS
booster is broken EF

(Bob Faster) #9

my booster was full of fluid but the seal was not compromised, no holes, no rips, still pliable, I didnt know if it was a failure of the sleeve in the booster or from the master. so I replaced both at the same time, much like Geo, decided it was cheap insurance to do it all.

(Karl) #10

Andrew, I also am trying to learn the precise function if this system. What is the activating component that allows the rear chamber of the servo to gain atmosphere?

(randy olson) #11

Karl, I believe the “activating component” that allows the rear chamber of the servo to “gain atmosphere” is the reaction valve. The simple test to demonstrate how it works is in my original post: “I tested the reaction valve and it seems to work. I started the engine. I took the hose off the plastic cap and there was good vacuum. When the brake pedal was pressed, the vacuum stopped.” The reaction valve has a vacuum line connected to the rear chamber of the servo and provides vacuum until the brake pedal is pushed. When the pedal is pushed the vacuum is shut off, allowing the master cylinder to apply hydraulic pressure to the brake lines.

(Tom D) #12

Karl to continue on what Randy said, the reaction valve is a two part valve. Vacuum is always applied to the front of the booster. When the brake pedal is at rest, the reaction valve connects the vacuum from the back reaction valve port to the front port, and then directs it to the back of the booster, thus vacuum on both sides of the booster diaphragm, thus no boost. When the pedal is first pressed, the reaction valve poppet moves and first closes off the vacuum from the reaction valve back port to the front. As the poppet moves further, the poppet opens the connection from the front of the reaction valve that is exposed to atmospheric pressure to the front reaction valve port connection, thus directing atmospheric pressure to the back of the servo, thus adding boost. The reason it is called a reaction valve is what also happens at this time. Note that when the atmospheric pressure enters the chamber and is directed to the back of the servo, it also is directed against the reaction valve diaphragm, and as it pushes on the diaphragm, it tries to close the valve that is allowing atmospheric pressure to go to the back of the booster. This is what provides the operator with the proper brake feel and the ability to modulate the vacuum boost.

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(Karl) #13

I see!, so the poppet… ( little plastic piston) is the player in all this. My problem is I’m not getting this secondary function of push back. The peddle goes down, I get boost, but pedal does not want to return much.

(Harvey Ferris) #14

Good discussion above. The system is a little convoluted compared to modern power boosters but once you get your arms around the concepts, trouble shooting can occur in manageable doses.
One very imperfect test I have done. The vacuum reservoir tank on the passenger firewall stores enough reserve vacuum to get you stopped once or twice, even when the engine is off. I have been able to start the engine to get the tank charged. Shut the engine off. Step on the brake pedal. it will be subtle but on my car I could hear the elements of the booster “wheeze” as the air pressure on the back of the diaphragm moved it forward. Not a conclusive test but if you hear nothing, that is probably not a good sign. The next step would be to rejigger the vacuum line from the firewall to apply vacuum directly to the front side of the booster. Leave the rear port open. (engine running as a source of vacuum) This should activate the brakes. (jack a wheel up and see if it locks up as vacuum is applied). Next is with the standard hose configuration, remove the reaction valve from the MC and push the plunger manually with your finger (engine running as a source of vacuum). If this activates the brakes then the problem is most likely the little plunger in the end of the master cylinder. I have tested it by holding a metal plate over the end of the MC and having someone step on the brake pedal. You’ll hear it ping against the plate. Or with no plate, it will shoot across the room:) It should also retract when the brake pedal is released. Also, at some point, evaluate that you are actually getting vacuum. A finger over the end of the hose will give a rough idea. These tests cover the major elements of the power boost part of the system.
Also, a little know safety feature. Even if the MC is drained of fluid, the parts are configured so when you step on the brake pedal, the internal rod pushs on a little T shaped plate in the end of the MC, which pushes out the little metal piston manually. A one shot hail Mary deal that will activate the brake booster and get you stopped. Once.

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(Tom D) #15

Harvey, as long as the engine is running and you have vacuum, it will not be once, but indefinitely. If engine off, it will last several time until the vacuum reservoir is empty. Ever wonder why the vacuum reservoir is larger than most? That may be why.
Tom

(Tom D) #16

Karl, think what would happen if the reaction valve did not have the diaphragm. As you press the brake pedal slightly to give moderate braking, the poppet would move slightly also. This would just open the atmospheric valve a little bit, for illustration, say 1/16 inch. This would not allow atmospheric pressure to be in the back of the booster instantly, BUT, if you would try to hold moderate brake pressure, the atmospheric valve would be held open this 1/16 inch, and over the next second or two, enough atmospheric pressure would enter that soon, the back of the booster would be at atmospheric pressure and thus full boost, thus basically full pressure, thus locked brakes, even though you only pressed the brake pedal a little bit. What does occur, is when you pressed the brake pedal slightly, and opened the atmospheric valve the 1/16 inch, the atmospheric pressure started to enter in the booster and against the reaction valve diaphragm. It pushes the reaction valve diaphragm and closes the atmospheric valve, not at 14.psi, but lets say at 3 psi, trapping only the “wanted” 3 psi of pressure in the booster, adding only moderate boost, not full 14 psi boost. If you now push the brake pedal a little more for more braking, the poppet moves open again allowing in more air pressure, say 8 psi. Again, the 8 psi pushes back on the reaction diaphragm more, closing and trapping 8 psi of boost in the booster. If you want max braking, you push as hard as you can, opening the atmospheric valve all the way, keeping max 14 psi boost on the booster and that is the max the booster can help. Any additional braking is through the hydraulic circuit. (which as Harvey alludes to means if one loses the m/c hydraulics, max back up braking is limited to max boost braking.) It is this reaction valve diaphragm that allows you to modulate the vacuum pressure, thus boost, and not just have all or nothing.
Tom

(Karl) #17

I see, so it’s mostly a modulator. Not just a valve. Now I’m getting it. So I’m wondering now if my brake drag could just be a balky rebuilt master combined with a somewhat balky poppet. Or if I have a leak in the system somewhere not allowing enough " push back" from the RV.

(Harvey Ferris) #18

Tom, the reason I said only once is that for a MC drained of fluid, there is no “suction” to pull the activation pin back when the pedal is released. So in this extraordinary situation, which I have been fortunate to never have experienced, I suspect the brakes will be locked on at full 14 psi, even as ones foot is taken off the brake pedal. You are correct that the vacuum reservoir should give you more than one activation of the power booster, in other scenarios such as the engine simply shutting down unexpectedly at speed.
Of course, the return of the activation pin, or more precisely, it refusing to return, can be a cause of the brakes being locked on.

(Tom D) #19

Harvey, as I was saying to Karl, the atmospheric pressure pushes the reaction valve closed, pushing the activation pin back into the m/c, deactivating the brakes. They should not lock Suction does not pull the pin back. And, since the topic of the brake backup system is being discussed, this gives me the opportunity to emphasize that the car should not be driven with the vacuum line disconnected except for testing, as that eliminates the failure backup.
Tom

(Harvey Ferris) #20

Tom, my memory tells me I was once upon a time exercising the brake pedal back and forth and watching that pin pop out and then retract. But I wouldn’t trust my memory that much!

(Tom D) #21

Harvey, I am sure your memory is correct, it probably did as you say. But when operational, the reaction valve and the small piston work against each other.
Tom