Broken brake booster?

[PepijnWK]

Hi,
I have a brake problem. Last weekend I drove with my rover (3500 auto 1976). And the brakes were dragging not only dragging but at some point they were stuck. I released the vacuum hose from the manifold and eventually the brakes freed up.
Since then the brakebooster does not seem to work anymore. The car brakes only when I apply a lot of force on the brakepedal and the brakes are almost locking directly then.

So what I think is that the brake booster has had it.

Is that assumption right?

My master cylinder is actually renewed two years ago and it is a dual line system:

my brake booster actually looks exactly like this so two ormal pressure lines va metaltubing to t-piece thas is connected via tube to master cilinder airvalve.


Two vacuumlines connected via t-piece to Manifold and backend/underside of master brake cilinder.

 

[PepijnWK]

Well this is really perfect........I took the airvalve apart and everything looked ok. ( should because the masterbrake cylinder is only few years old.) Drove a few miles just to see how things were and it drove ok. But when I wanted to drive it into the garage backwards I noticed it deed not "creep" on idle.
Forward just a little but not backwards. I noticed a hissing sound from the airvalve. When I took the air valve out and pulled on the rubber the vacuum disengaged. Vacuum was very strong. but it locked the brakes....... It waas only after I pulled the vacuumline from the masterbrakecilinder the brakes released!
So I think there is something wrong with my master brake cylinder!!
could it be that the rubber in the circle is stuck somehow??

 

[amcdonald]

Hello. I had seemingly the same issue, immediately after I sent my dual line MC for rebuild. When I disconnected the vacuum hose the brakes worked fine (with no boost!) but as soon as the booster was connected they were not releasing.

I researched the mysterious plate in the MC (plenty of duff info on line) and the pin / plunger you identified. I found the pin was not smoothly moving in the bore and I figured it was sticking so I rolled up some wet and dry and poked it in and slowly smoothed out the bore surface tediously after going through a lot of wet and dry roll ups and that fixed it.

Then I had other issues (still) with this horrible dual line setup but sticky brakes is not one of them!

 

[PepijnWK]

thanks for your reply. "Wet and dry" is emeraldpaper, sandpaper I guess? I will take a look today to see if I can get the little plunger to move today.

edit: Just looked at the little plunger/rubber in the front of the MC that actuates the airvalve. That seems to work normal! I can push it back with an allen key with no force. Feels about right. Slides smoothly down. and upon applying the brakes it moves to the front.. So that seems to be in order....

in case you wonder why the endplate of the MC is not looking like new. That is because of the fact that I used my old endplate because the vacummtube on the new one is straight and does not hav the bend in it like the old one does.:

So could it be that if the lines (with atmospheric pressure, not vacuum!,) have leak, that that may cause sticky brakes?

Somehow the amount of vacuum might be so high that the piston in the master cylinder is not returning fully?

any thoughts?
edit:
well i have my own thoughts: I think that the pressure on the oher side of the membrane should be somewhat below atmospheric pressure. If it is at atmospheric pressure then why bother to connect the "atmospheric pressure" sides (from MC en Booster) with a steel tube? A simple filter would suffice then.

So if the pressure of these lines should be lower than the normal air pressure outside the system and a leak occurs the pressure raise to the pressure outside the system. The difference between the two sides will increase resulting in slightly pulling on vacuum sides resulting in breaking or at least not fully returning to the "rest" position.
Result is sticky brakes. Started by applying the brakes and getting worse each time the brake is applied. This is what I observe in my case.
Does this makes sense?

so what I think is most logical is a leak in the pipes connecting the master cylinder and the brake booster. I also did not use any clips to secure the hoses. Which I did not really give any attention because I thought that normal air pressure would be what was needed.....

Well could be but I did some further investigation. I disconnected the booster from the vacuum line but the master cilinder connected to vacuum through the air valve. No sticking.

My conclusion so far is that the brakebooster is faulty... But what part of it? I thought that mostly the airvalve is the culprit at least the smal rubber moved in the piston by the valve. But at first glance that looks ok and moves freely.....


The dual line is more safe but would not it be smarter just to convert to single line brake system?

It is easier to bleed, what I hear from people less troubles and apparently better breaking due to the fact is has less problems....

But that means getting a single line MC single ling brake booster and manufacture and reroute brake lines.
what does one think?

 

[PepijnWK]

Not many ideas.....
But I finally think I got how this system actually works. That is why my posts might not have been understandible in the first place because I did not understand how it worked.
But here is what I think:
There are two membranes one in the booster a big one and one in the airvalve. these bothe sides are connected to each other. So lets see one side is connected to the vacuum line of the manifold.so is the back side of the airvalve. the other side of the airvalve is connected to the other side of the big membrane of the booster.
These are in an equilibrium with each other. But as soon as you brake the airvalve is pushed open by the little rubber of the master brake cilinder and air rushes in helping pushing the piston in of the brake booster and thus the actual braking.

If this is how it works the airlines rubber and steel should be airtight. As soon as they are not the equilibrium is distorted and braking function also.
So it is very important that the airlines are airtight.
I think that is what is wrong with my system. The connections of the tubes are not really airtight.
To be continued.

 

[Barten]

Hi Pepijn,
I have the same system as you in my 3500s. In my other Rover 3500, i have a single line system. Which has a much more "modern" feel to the brake pedal. Brakes immediately and progressively. I my 3500s I need to really stamp on the brake for it to work properly and it is tiring in the long run. I think there is something wrong as well, but dont now what. It seems like the vakuum assitance is absent. I will be glad to see if you make some breaking progress!
Regards, Barten

 

[sdibbers]

@PepijnWK youre pretty much right in your understanding of how the servo functions. The small piston opens the valve in the ‘flying saucer’ when that opens atmospheric pressure pushes the large diaphragm in the big canister to assist in brake pressure. If the air valve I’m the flying saucer doesn’t seat properly when you release the brake pedal the brakes can drag or even lock on.
I’d check the surface of the metal base of the flying saucer to ensure the rubber diaphragm is sealing against it. Also, is the spring present still under the cap of the flying saucer? If that’s missing you can have the valve stay open.

One other thought: if you’ve replaced the master cylinder check that you’ve correctly adjusted the pushrod going into brake pedal. If it’s too long and the pedal is too high it can stop the master cylinder piston from returning to its rest position which can cause pressure to stay in the brake lines. That can cause dragging along with activating the servo.

 

[PepijnWK]

Thanks for the replies! I made sure every connection is airtight now and no sticking of the brakes anymore! That is a plus! Braking is better now but still not as good as modern car. My volvo 265 from 1980 brakes a lot better.... But for now it is driveable. I still think that the single system is better to manage but then I have to buy a lot of parts and the securtiy of two systems is nice to have. But the braking is not as strong as i would realy like it. It is a lot better altough now!

 

[sdibbers]

Thanks for the replies! I made sure every connection is airtight now and no sticking of the brakes anymore! That is a plus! Braking is better now but still not as good as modern car. My volvo 265 from 1980 brakes a lot better.... But for now it is driveable. I still think that the single system is better to manage but then I have to buy a lot of parts and the securtiy of two systems is nice to have. But the braking is not as strong as i would realy like it. It is a lot better altough now!

I have a dual system on my 2000TC. It feels pretty modern for brake performance, but it took a fair amount of work to get it there. Here’s what I did:

(1) Hand brake cable adjustment. If you find that the cable has been over adjusted and doesn’t allow the rear quadrants to return when released the rear brakes can not self adjust. I’ve seen a couple of cars where this has been done. Release the cable from the rear quadrants and try manually moving the quadrants while listening for the adjuster clicking inside the calipers. You may find you have to go for a drive like this to ‘kick start’ the adjusters into working again with the great pedal pressure and heat soak from the brakes. When you feel it’s adjusting replace cable and adjust so there’s no tension on cable when handbrake is released.

(2) Are the front brake caliper pistons rust free and moving smoothly? I found I had partially seized pistons on my car that caused a slow release after releasing the brake pedal. I replaced both the pistons and seals and was amazed how much of a difference that made. I wouldn’t have said the brakes where bad before - but a huge improvement.

(3) Make sure the serrated wheels for the automatic adjusters in the rear calipers are working correctly. You can just about take the covers off the calipers in situ above the diff. Remove those and get someone to pump the pedal or pull the head brake. You should be able to see if the wheel is moving, even if it’s not clicking to the next stop. If it’s not you can try using a small screwdriver to see if it stuck - don’t go crazy, it’s very easy to over adjust the rear brakes. If it’s seized your best bet to to remove the calipers, reseal the cylinders while you have them out If you do. Unwind the mechanical piston from the adjuster wheel and clean/degrease thoroughly. Before reassembling use a tiny amount of 3 in 1 oil or similar to lightly lubricate the adjuster mechanism.

Test all of the above in order, if it’s working better after (1) go ahead and enjoy the car. Same with (2). I’ve tried to order these in time commitment, (1) being easiest and (3) being a pain in the neck.