Brakes locking up when engine starts

[NogE]

Hi all...
As most know this is fresh rebuild..so totally new dry brakeline with remote servo....New lines from master cylinder to inlet on servo..and outlet off to brakes.
All blead brakes good pressure..
But soon as I start engine all four brakes lockup...
I've adjusted the rod to pedal from cylinder to max..no luck,..soon as I disconnect rod to pedal they unlock...
Iv taken cylinder out to check piston is fully coming home..it is..mk1 68...
Any thoughts chats...

[DougT]

I know you said that you have bled everything successfuly, but I do wonder about the pipe between the master cylinder and the remote servo. This is only because I cannot think of anything else....

Is this a new fitted servo or an original fitment ? As a general rule the servo should point upwards slightly to help bleeding but if it is original then it should be mounted OK. It can be tricky to bleed between the master cylinder and the servo on some installations as there is no convenient bleed nipple on that part of the brake system. On some cars, it is only possible by loosening the servo inlet fitting and this gets fluid everywhere. So have towels handy and wash off any fluid that gets on to paintwork etc.

Might be worth a try.

Doug

[NogE]

Hi Doug...yes I did do this..as part of getting fluid all round I did take that pipe out of the inlet side..and did get abit there to get things started...
It's a remote servo...it is point upwards slightly....it is the old unit and was idle for about 10years pos more...
Can these seize?.. diagram maybe.....see pic of servo

[NogE]

I did take diagram apart to clean....I never took the cylinder side of remote apart tho

[DougT]

I have found a useful set of diagrams here:

http://www.mgb-stuff.org.uk/servo.htm

which explain the operation of the remote servo. So with engine running but no pressure on the brake pedal the vacuum needs to get to the back of the servo as well as the front and this is controlled by the air diaphragm. I think I would have a check that you are happy with the air diaphragm unit as any air leaks would seem to cause the brakes to come on. But that doesn't explain how disconnecting the pedal helps.

Doug

[NogE]

Great bit of info there Doug...I'll be checking that today...I'm sure this will help others to...

[NogE]

So new cylinder in.....same prob
Guys... if i disconnect the vac pipe to manifold They DO NOT lock....soon as I connect back they lock..

[DougT]

assuming you have a flexible pipe from the air diaphragm unit to the back of the servo, just wondered if you have a convenient t-piece of some sort so you can connect vac pipe from manifold to both front of servo and directly to the back without the diaphragm unit. Brakes should not be locked like this and would point to diaphragm unit being at fault somehow.

If brakes are locked with direct connections to front and back then you have a leak in the back section of the servo which probably means new servo is needed.

Doug

[NogE]

Thanks Doug...rain stopped play today...I'll be back out there in morning...great help mate..thank you

[NogE]

Just thought Doug....
So after running,they lock up...
They will also release slowly when engine off ..not doing anything, just leaving them as is after running....

[DougT]

There is probably a one way valve at the vac connection on the servo which means that vac is being maintained at least in the front part of main chamber. Normally there is a bit of leakage somewhere so vac disappears over time which would release locked up brakes.

I think it points towards the back of the servo being OK but it would be good to check that just to narrow it down. Just thought that you might need to stick your thumb over the diaphragm outlet as otherwise you might lose the vacuum in the front section.

But that gives another idea. If you disconnect the back section from the diaphragm unit and run the engine, then you should be able to feel if the diaphragm unit has a vacuum from it's output. It should do and if not then there is something not right with it.

Doug

[NogE]

A guy on buysellcortina sent me this link incase I hv to do service on the servo...
http://www.triumph-spitfire.nl/servoimages.htm

I must say ,this forum is great...responses are fast..wish I had come here FIRST off ..instead of that mk1 mk2 mutley/Fred plot... all other there are ok..tho I wish I'd never ment that mutley twat... ...

[DougT]

Yes that is a great set of pictures. I hadn't realised that you could get to the diaphragm in the main chamber, All the ones that I have seen have had the main chamber sealed. If yours is the same than it can be repaired. And these are from the Ford parts book.

E2.30.01[1].png

E2.31.01[1].png

Doug

[NogE]

Hi all,

Its sorted...turns out the small diagram was not letting air past the air valve was stuck shut..soon as I released it the main large diagram let go..fix now..thanks for all yr help and advice people....yr all some very helpful here... great forum Luke..

[DougT]

Great result and thanks for updating the thread with how you solved it.