Sticky brakes. immediatly loose when engine is switched off, Solved!

PepijnWK

Active Member
#1
I have a problem that the brakes are sticking ! When I shut down the engine the sticking is gone. It has to do with the vacuum somehow I think.

I have a 1977 P6 3500 with dual system.
I made a drowing how it is assembled. It is just alittle bit differenty than I saw in the Haynes manual:


Let m,e explain this amateur drawing:
high Left corner is the master brakecilinder. Ther are two pipes leaving the master cilinder.

Number 1 from the front of the plastic reactionvalve. This hose divides into two hoses which are connectet each to metal tubes positioned under the engine and ending at the front outer tubes from the servo unit.
number 2 from the mastercilinder (just before the reactionvalve ) directly to a t-piece which iut connect it to the inlet manifold and with the tubes going to the back of the servo unit.

I hope this makes sense....

The difference with the 1974 setup is that my servo has two entriws for the hoses oming from the reactionvalve.
Second the hoses from the back of the servounit are bothe connected to the vacuum from the manifold and dirctly to the mastercilinder via the t-piece.

the 1974 drawing shows that the two hoses from the back of the servo unit are sperate and not connected via a t-piece. one hose to the master cilinder and the other to the manifold.



Could the way my vacuum hoses are attached be wrong? Or might there be a leak some where causing my brakes to stick?

thanks in advance for your answers!!
 

Tor

Active Member
#2
It looks like the three different T-pieces have been added to compensate for one failing component. The one on the tube from manifold to booster is linked with the rear of the reaction valve to boost the return pressure on the valve. The basic symptom sounds like the valve is the issue, but with all the T-pieces the whole system is upset now. The valve can fail to operate correctly because of a stiff seal on the piston (in the forward end of the master cylinder) that actuates it, and/or because of the spring in the cap being too weak to push it back when foot comes off pedal.

I would first delete all the T-pieces and make sure all connections are according to the manual. Then blow through all the rubber hoses from both ends to make sure they're all clear. Remove the reaction valve cover from your master cylinder and inspect the diaphragm and spring. The first thing, if the diaphragm seems intact, is to add a second, identical spring. Or you can stretch the one you have a bit to provide more return pressure on the diaphragm.

If your booster bell really has two metal pipes on it, in addition to one open plastic connector and one non-return valve on the rear of the bell, plug one of the metal pipes.

I have the same system and spend far too much time being annoyed with it. When I fitted a NOS seal kit to the master cylinder, the new seal for the actuating piston was far too stiff and I ended up reusing the old one.
 

sdibbers

Well-Known Member
#3
All good points above. If the brakes are still and free off after the engine stops I would also check the air valve (flying saucer on the end of the master cylinder) for leaks. Also check the brake pedal is at the correct height, if the pushrod is adjusted to be too long it can activate the valve too giving those symptoms.
 

PepijnWK

Active Member
#4
thanks for the replies guys. The brake mastercilinder was rebuild as was the servo. The t-pieces although not seen very often are what I think original! check out these illustrations of the parts catalogue:



so they seem to be original. I just look at the illustrations today. If I had known this I would not have bothered to make my drawing.........

but still the brakepedal hight might be an issue since I have never really checked the height..

The spring in the reaction valve was missing!
does somebody has a picture form the original? |I just put in a spring |I could find laying around.



this might improve things!! although i would think this spring ensures that the vacuum is being secured and that is what might cause the sticking..
 
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Tor

Active Member
#6
I’m learning something today - there was an update to the dual line system! I’m struggling to understand how all the interconnections will affect the workings of it all, but yes you must have a spring in the valve for the boost to come off again properly. I’d love to hear more about how your pedal feels afterwards.

Does your car have part no. KRC1071, “non-return valve and adaptor”?
 

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PepijnWK

Active Member
#7
I’m learning something today - there was an update to the dual line system! I’m struggling to understand how all the interconnections will affect the workings of it all, but yes you must have a spring in the valve for the boost to come off again properly. I’d love to hear more about how your pedal feels afterwards.

Does your car have part no. KRC1071, “non-return valve and adaptor”?
Part krc1071 is not present on my Car but the part 13H9216 which connect the tube to the manifold has a non return valve in it. It is a one way only, preventing stuf getting into the manifold.

My brakes have to be bleed seriously as well because the first two "pumps" there is no braking. I hope to be able to bleed the system this weekend.

you stated: "you must have a spring in the valve for the boost to come off again properly." But how I see things the spring in the valve is not for the boost to come off but to keep the boost on or the valve closed ready to be boosted.

in the next figure you see the spring is pushing the valve colesd thus preventing the vacuum to be cleared.


the right side is vacuum the left side just atmosferic pressure. So when appling the brakes the valve is pushed or sucked towardes the vacuum. The spring helps this action and not the other way around. So in theory I would think thgat the spring in the boostere might be a little to weak......But in my opinion this spring was pretty strong.

So I think possible causes might be leaking pipes some how and pedal height might also help.

But I am just thinking things out in theory and am I am not experienced so I am happy with other ideas or suggestions!!
 

Tor

Active Member
#8
You seem to understand this system better than I do, the only thing I'm fairly confident about is that the 'reaction' of the reaction valve is to actuate boost, given current or residual (engine off) vacuum. Every time the pedal is depressed. When the spring returns the diaphragm, and the piston thus returns into the m/cyl there should be no boosting of brakes going on anymore. If it does not close, and this (as discussed several places on here) was an issue in early boosters - single and dual line - that the reaction valve piston couldn't readily return, leaving the brakes to bind ON with the boost that was present in the system. This because there was no spring fitted behind the valve cover on early boosters. Someone else will have to correct me...

I'll be keen to hear how it works when you've bled the system again, now you've fitted that spring.

I mean, seriously. I wish I had studied engineering. These cars are complex!
 

PepijnWK

Active Member
#9
this afterrnoon I tested the brakes by starting the engine en firmly pressed the pedal a few times. Then leaviung the engine running tried to move the car ( being in neutral) and it was not possible brakes are blocked. With the spring in the reaction valve..... So the spring did not make the difference.

So I took out the airlines to inspect the metal pipes and I will replace the rubber tubing.

what I found is that the both halves of the brakebooster were not really tight together. The bolt might have come lose over time.....
One of the iron pipes had a leak! it was very small so I doubt whether this is the problem but hey soldering it will at least improve the working of the brakes!
it might be all litlle problem which causes in the end the brakes to be stuck....



 

PepijnWK

Active Member
#10
today started with repairing the iron tubes with silver solder.:






so three little holes are repaired now.

I allso bought new 10 mm hose so everything nice and new !!

I still think it is something within the booster cilinder that causes the vacuum to hold.
 

sdibbers

Well-Known Member
#11
The vacuum should hold on both sides of the diaphragm inside the servo. If you have a leak letting atmospheric pressure into the back of the servo you’ll get the problem you have. The areas to inspect for that are:

The fit between the front and the rear housings on the servo body.

The plastic air valve on the end of the master cylinder.

The piping between the air valve and the back of the servo body.

I would also add if the brake pedal is poorly adjusted it can cause the air valve to not close fully giving the same problem.
 
#12
The tiny leaks in the pipes could well produce the symptoms. I'd bet a penny or two that you've got the root cause there. Maybe I'd even go as high as a nickel.

Yours
Vern
 

PepijnWK

Active Member
#13
thanks for the replies!! Well don't place the bets yet........

I repaired the pipes and replaced the tubing with new ones. But the problem persists.

I took away the airvalve for inspection but all seem to be in order.



I also took away the valve housing en the little piston. But all seem to move freely and looked ok.

What I found out the I think the airvalve is not airtight mounted on the maseter cilinder. I couyld hear some hissing.

I will look into that problem next.

If that does not help I will look at the servo unit. May be to stretch the spring al little or something .

About the pedal. The pedal height is so that the pedal has free travel for about 2 a 3 cm. so the pedal is not giving pressure already when it ios in its starting point
 

PepijnWK

Active Member
#14
this morning dismantled the reaction valve . I made a new gasket and applied very little bit of gasketsealer. this helped. The hissing round the reaction vale is gone! but so is the braking!! I have no pedal prssure whatsoever. in the beginnen I had a little bit but now it is gone........
So I will try to bleed them and see also if I can find a leak or something!
Old Rover ...........keepp you posted


 

cobraboy

Well-Known Member
#15
You may find you can bleed by undoing the outlet pipe at the servo and have an assistant push a bit out, you may save a full bleed.
 

PepijnWK

Active Member
#16
Well tried to bleed the brakes today. Failed miserably. Could not get any pressure. even out of the mastercilinder cam no pressure! Verdict faulty mastercilinder. Although reconditioned two years ago ( not by me but by a friend) it is noit workingh. Could be that although it was reconditined the bore was to bad or something. So I ordered a new mastercilinder today and hope it will be better after installing that!
 

PepijnWK

Active Member
#17
So after lots of fun with the brakes I decided to buy a new master cilinder. They are not very cheap though.......

But I wanted to ask you : do you have tips to bleed the braakes? I have the nice dubble system with this nioce piece:



the pressure failure switch.

This will probably hinder my bleeding will it not.

In the manual it is stated that you have to bleed front and back at the same time. Well tht is not very convenient. You will need three persons to bleed the brakes. or should I block the switch somehow? I do not remeber where but I thoughty I read that somewhere. They stick a little screwdriver in the hole from the swithc on the failure switch. This prevents the little pistonlike part to move.

Any ideas are welcome!!!!
 

Tor

Active Member
#18
Yes, the shuttle inside the block must be "arrested" in the middle position so it doesn't slide left or right during bleeding to block one or the other circuit. That can be done with a suitable pin or electronics screwdriver held in place with an elastic band or something.
 

PepijnWK

Active Member
#19
So I ordered a new master brake cylinder for dual line setup:



thing was not cheap but glad that someone could sell it to me. (195 pounds....excl P&P)

things on top are obviously different than standard.
when i take of the first screwq i see this:



take it out:



springloaded thingy with tiny hole in the middle:



does anybody form you know what that is. Should i keep it in?

I have to find out how to connect the pipes because the fittings are definitely different and quire a lot bigger!:



Has anyone fitted a new mastercilinder like this and what connections did you use?
 

Barten

Active Member
#20
Hello Pepijn, i bought a new dual master cylinder from Wins 3 years ago, and I swapped out the new connectors and put in the old connectors. So I can use the original brake lines and connectors. I seem to remember the plastic thingy and the spring. They Are supposed to be there. Isnt there one in your old master cylinder? You will compress the spring and press the plastic thingy down, but it is like that AFAIK. In the end I got my brakes working, it was really har to get the air out of the line to the rear, but i cracked open the connection from the 5 way and back, bleeding each step. And I finally got fluid out on the aft bleedscrew. My brakes works now, but not as good as in my other V8, which has a single Line system. Regards, Barten
 
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