P/S Cap leaking

The cap on my p/s reservoir is leaking. I think it's because it's not tight enough. Is there a new cap gasket I can use or one I can install on top of the old one? Will I have to replace the cap? If so, does anyone know the part#. Thanks.

[immortal_suby]

The cap is vented. Do you have too much fluid in there?

What I did notice is when I backed off the cap a tad it got tighter, and my p/s was quieter.

[Earthworm]

What I did notice is when I backed off the cap a tad it got tighter

That works good for radiator caps as well

[DavieGravy]

This must be a somewhat common problem. I get a nice ring-shaped stain on the resevoir right around the cap. You might check an auto parts store, although I'm not sure if the part's a standard item.

[Trevor]

immotal suby,

You say that the cap is vented. This is contrary to what has been posted with reference to the effect of the O ring leaking air.

I have just examined mine and can see no evidence of a vent. The spring retaining arrangement may however give the impression of a pressure controlled vent.

A leaking cap would probably let in air and result in airation of the fluid and a noisy PS pump relief valve as does a leaking O ring and the reference by billisa to noise appears to be confirmation this.

Could there be a variation between models ? Your further comments could assist in determining if this is the case.

[Beav]

The cap should be above the normal level of the fluid, so aeration should not occur as a result. Typically p/s caps are vented in order to allow for expansion and contraction of the fluid as it changes temperatures. Otherwise the fluid could blow out the reservoir when hot and/or suck in air when cooling. There may not be a visible sign of venting or a purge valve, etc., but there's always a means for venting to occur.

[svx_commuter]

Hi Trevor,

I believe there is always air on top of the fluid level in the reservoir tank. The fluid level is a ways down form the point were the fliud would reach the cap. The cap may not let presure out and then it would build when the fluid heats up but it may also allow air to enter when the fluid cools off so as not to pull a vacuum on the tank.

The o-ring is below the surface level of the fluid and it's sealing the tube the fluid passes thru. When the fluid is very cold and viscous, there is a higher pressure loss as it flows into the tube. When the tank is at atmo press the pressure in the tube is lower so the outside air pushes past the o-ring to the low pressure in the tube.

At first I thought this was about the reservior on the master cyclinder.

Trevor seemed to describe my situation rather well. So, does anyone know the part number for a new reservoir cap?

Originally posted by billisa
Trevor seemed to describe my situation rather well. So, does anyone know the part number for a new reservoir cap?

Hmm.. 34438AC040 Cap $29.95 retail.

[DavieGravy]

Would a bad cap cause under assisted steering at engine idle or at low rpms?

[Trevor]

Beav I agree with what you say but I have my cap in my hand (Do I have to go cap in hand ? ! ) and I can not see how it can vent which is what has provoked my interest.

The top outside of the top is covered by a label and there is no evidence of any aperture in the top. The gasket seals the entire circumferenca so that any outlet must be within this area and I can find none. The centre stalk with calibrations has a hole at the tip but this is blind and appears to be a moulding issue.

But wait, could something unseen open here when the cap is in place and the spring arrangement is under load ? Checked this but no air will pass unless we are talking PSI above my blow rate ! Again talking this sort of PSI, I have even kissed the tank but could not find any evidence of a separate vent. However I can vouch for the fact that PS fluid is not my cup of tea ! Bloody horrible and only taken in the name of science and SVX stupidity.

This subject has interested me since reading various posts regarding THE 0 ring. A reservoir is normally purely a back up supply and which as you say is usually open to atmospheric pressure so how come an O ring at the outlet which does not leak fluid apparently lets air into the system ?

SVX commuter I note what you say in this regard but is an inch or two of pressure difference enough to make the O ring leak air inwards but not let fluid out ?

Firstly we must establish that in fact there is a vent and if there is where it is.

Now this is both an interesting debate and an investigation of fact and as such is a fun and is purely an amateur exercise so that the influence of ca$h should not become involved. If it does I am out of here.

[Beav]

I'll have to take a look at mine the first time my memory kicks in. Maybe the SVX's cap is too tight and that's why it has a pattern failure of the 'o'-ring, where most cars do not. I do know that most cars, if overfull, will heave their fluid when the wheels are turned side-to-side while the engine is off, so there has to be some sort of allowance for breathing. It could be a deception, that it doesn't actually seal as well as it appears, especially in billisa's situation.

[svx_commuter]

Originally posted by Trevor

SVX commuter I note what you say in this regard but is an inch or two of pressure difference enough to make the O ring leak air inwards but not let fluid out ?

Hi Trevor,

Consider the difference in viscosity of air and ATFluid. The air will flow very easily thru a small crevasse as compared to the ATF with the same pressure differential.

Now consider that the pressure differential is greater when the fluid is flowing thru the suction passage and that the higher pressure is on the outside of the tank.

This happens to be a common problem on pumps with suction pressures below the atmospheric pressure..

Take care

[Trevor]

I agree with all thoughts so far which are in line with my own questions regarding this apparent problem.

Therefore it still appears to me that the cap is not in fact vented which as we agree is strange. Surely it was not designed on the assuption that it would leak.

Now billisa says that when the cap is backed off and a better seal established the PS was quieter. My PS made a noise prior to replacing the O ring which I presume was the relief valve chattering due to airation. I intend to remove my cap and note the result.

So far to me everything indicates a closed reservoir which is not open to atmosphere and this is indeed strange. How the hell does it replenish any fluid loss ? Air must have to get in somewhere.

Now billisa I hope you are watching all this as you started this bouncy ball rolling