Faulty brake servo?

[NickandNeil]

Trying to get my Exmo ready for the IVA.  With my foot on the brake pedal, when I start the engine the pedle only gives a little.  If I apply the brakes hard I can't lock the wheels.  Have been told it probably needs a new servo as the car has been standing for twenty years.  Are they reapairable?  It is off a 1984 Sierra.  Should I just dump it and start again?  If so any recomendations off the type i should get, the existing one is very close to the Exhaust manifold.  It has a slave cylinder bolted to it by two bolts?

[Guest Livvy]

The Servo works on a vacuum and to see if it is working correctly remove the pipe that comes from the inlet manifold and see if there is any difference at the pedal - If there is no difference check the pipe is connected at the other end and not perished and a further check is to put your thumb over the end of the pipe if all is good you will feel suction - If you do and you don't feel any real change at the pedal then its a safe bet that the servo needs changing - I do not believe they are repairable - If none of the previous is true I would give the rest of the braking system a full service 

Cracking video below that explains the servo workings 

 

 

[richyb66]

The servo isn’t repairable but if the pedal drops when you start the engine then the diaphragm would seem to be OK. Plenty of people have used old servos and I can’t think of many occasions when they needed to be replaced due to a fault so I think it’s unlikely to be a problem as they’re generally pretty resilient.

When you say you can’t lock the wheels, is this actually driving the car? If the brakes don’t stop the car then you maybe need to be looking at other causes. Even without the servo you should still get decent braking even if the pedal effort is high. Are the pads new and do they need bedding in and are the brakes bled thoroughly? Not that it’s likely to be a cause, you say the servo is very close to the manifold, how close it is? Is the pedal firm and doesn’t creep down if you stand on it really hard?

All of the Sierra master cylinders are 2 bolt fixings so that’s not conclusive, the angle of the 2 fixings relative to horizontal vary depending on the master cylinder type and the servo is matched to that. Swapping the servo and master cylinder isn’t a 5 minute job so be certain it’s the cause before committing to it.

[NickandNeil]

On 7/29/2019 at 10:46 AM, richyb66 said:

The servo isn’t repairable but if the pedal drops when you start the engine then the diaphragm would seem to be OK. Plenty of people have used old servos and I can’t think of many occasions when they needed to be replaced due to a fault so I think it’s unlikely to be a problem as they’re generally pretty resilient.

When you say you can’t lock the wheels, is this actually driving the car? If the brakes don’t stop the car then you maybe need to be looking at other causes. Even without the servo you should still get decent braking even if the pedal effort is high. Are the pads new and do they need bedding in and are the brakes bled thoroughly? Not that it’s likely to be a cause, you say the servo is very close to the manifold, how close it is? Is the pedal firm and doesn’t creep down if you stand on it really hard?

All of the Sierra master cylinders are 2 bolt fixings so that’s not conclusive, the angle of the 2 fixings relative to horizontal vary depending on the master cylinder type and the servo is matched to that. Swapping the servo and master cylinder isn’t a 5 minute job so be certain it’s the cause before committing to it.

Thank you for the advice,  the front brakes are discs, but not massivly large ones.  Both had new discs and calipers one month ago.  The rear are brake shoes - which I had to get relined as no one stocked the old 1984 sierra ones.  We thought about putting discs on the back but after reading on one of the forums that alot of sevens fail the IVA because of overbraking, decieded to stick wuth the drrums for the present.  |The car seemed to brake sharper at first but we seem to have to press alot harder now to get the same braking.  We were braking hard on the road, sorry if that was confusing.  The pedal seems hard and does not sink when pressed withput the engine running.

[NickandNeil]

On 7/29/2019 at 10:46 AM, richyb66 said:

The servo isn’t repairable but if the pedal drops when you start the engine then the diaphragm would seem to be OK. Plenty of people have used old servos and I can’t think of many occasions when they needed to be replaced due to a fault so I think it’s unlikely to be a problem as they’re generally pretty resilient.

When you say you can’t lock the wheels, is this actually driving the car? If the brakes don’t stop the car then you maybe need to be looking at other causes. Even without the servo you should still get decent braking even if the pedal effort is high. Are the pads new and do they need bedding in and are the brakes bled thoroughly? Not that it’s likely to be a cause, you say the servo is very close to the manifold, how close it is? Is the pedal firm and doesn’t creep down if you stand on it really hard?

All of the Sierra master cylinders are 2 bolt fixings so that’s not conclusive, the angle of the 2 fixings relative to horizontal vary depending on the master cylinder type and the servo is matched to that. Swapping the servo and master cylinder isn’t a 5 minute job so be certain it’s the cause before committing to it.

Thank you for the advice, the Video was very informative.

[phaeton]

Can you take it to a local MOT garage & put it n the rollers? That way they can tell you the effort on each corner, some also include the ability weigh the corner.