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Strange Sound Like Compressed Air Escaping


Luke Skywalker
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I have a new issue to sort out.

In my TR4A, with OD, when I travel at high RPMs and then slow down and apply the brakes I'm at times hearing what seems to be the sound of compressed air escaping. Maybe something is rattling in the driver's side rear drum brake, but I've not yet been able to isolate the issue. I was even wondering if it's the pressure escaping from the overdrive unit, then building back up--but that's fluid pressure, not air (unless my fluid is low, I wouldn't think that could be the problem). Is there any reason to suspect the OD unit?

It's a bit like when you remove the air chuck from a compressed air line. A few times it seemed to be coming from my foot-well, driver's side, but then more from behind me.

This happens when the OD is not engaged. I've not even tried to listen while it's engaged.

Any better ideas?


Jedi Trainee
OD does not make compressed air noises. You will see on a parallel thread that I ran mine on a test-bed so I found out what goes on with these.

Have you got a brake-servo fitted? Not original on UK spec cars but lots have em.



Great Pumpkin - R.I.P
Just a thought: Could you make a recording of it for us to hear?


Luke Skywalker
Country flag
Andrew, it's a one-time pop and I hear it when I just start to brake. I'll go for a run tonight and listen closely again.


Darth Vader
Normally no brake servo on a TR4A although someone posted pictures here of a TR4A in the UK that had one a couple years ago.

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