Locks, vacuum, and brakes

[MacGeek]

I noticed today that the locks are not working, fully.  After the car has been running, they work fine.  After a while of sitting, they do not work.  (Which I now realize explains why I could not get into the trunk the other day.)

In searching I stumbled across this post but after reading it am confused.  I have the same car, '79 300SD.  To my knowledge I do not have any brake issues, they work just fine.

Are the brakes at all tied into the vacuum system?  I do not want to have brake problems.  I can live for a while with the locks, knowing that if I run the car for a bit they'll be fine.  On the other hand if this could impact my ability to stop, then I'll find a good mechanic and have them find and fix the problem.  In the next couple of weeks I am planning on taking a 800+ mile trip with my 3 year old daughter.  Needless to say I need to make sure the car is safe first.

A BIG THANK YOU to the other members  of this board.  There is lots of great information here and very helpful and knowledgeable people.

[Big_Richard]

diesel engines produce no vacuum, but the brakes, locks, climate control and shift modulation is all vacuum assisted/operated

How is this so?

The engine has a vacuum pump on it.

if the fault with your locks was insufficient vacuum all the time, youd have un-assisted brakes (rock hard pedal) and the locks wouldnt work if the engine was running or otherwise

this tells me the fault isnt with your pump, its infact with the central locking circut itself. There will be a slow leak in it, that allows the vacuum canister to fill with air gradually after the engine has been stopped for a while. Others know of methods for finding these leaks quickly and easily.

[s class]

Find a quiet place (maybe your driveway in the evening).  Sit in the car, run the enigine for a couple of minutes, with the locks in the 'locked' position.  Shut off the engine and listen carefully if you can hear a whistling sound from any of the doors.  Repeat this test with the locks in the 'unlocked' position.  Depending where the leak is, it could leak in either the locked position only, the unlocked position only, or both. 

[MacGeek]

diesel engines produce no vacuum, but the brakes, locks, climate control and shift modulation is all vacuum assisted/operated

If I gave the impression that I thought the engine produced vacuum, sorry.  I know the engine does not, instead it is the vacuum pump.  What I was implying was that with my doing things in the engine compartment, I could have introduced a leak in the vacuum system, by moving a cable out of the way, etc.

if the fault with your locks was insufficient vacuum all the time, youd have un-assisted brakes (rock hard pedal) and the locks wouldnt work if the engine was running or otherwise

this tells me the fault isnt with your pump, its infact with the central locking circut itself. There will be a slow leak in it, that allows the vacuum canister to fill with air gradually after the engine has been stopped for a while. Others know of methods for finding these leaks quickly and easily.

Thank you for clarifying the brakes and vacuum, i.e. brakes are assisted by vacuum but not reliant upon having vacuum.

[MacGeek]

Find a quiet place (maybe your driveway in the evening).  Sit in the car, run the enigine for a couple of minutes, with the locks in the 'locked' position.  Shut off the engine and listen carefully if you can hear a whistling sound from any of the doors.  Repeat this test with the locks in the 'unlocked' position.  Depending where the leak is, it could leak in either the locked position only, the unlocked position only, or both. 

Thank you for the suggestions.  I'll give them a try.