Power Steering/Hydroboost hiss

Just happened to me last night. I had to hit the binders hard to avoid some fast lane changers in front of me. After that, now my brake pedal is hissing like the hydroboost is bypassing. If you touch the brakes or steering it goes away but with no input on either it hisses. Any ideas?



I have only about 15,000 km on it and the fluid seems OK. . I just Blacktec'd it a month ago, would hate to have to go grovelling to the dealer. .

 

Talked to the local tech that I trust and they talked me through the routine checks I already did. They figure its just an air bubble in the system that should work its way out. I sure hope thats the case. Anyone have this happen?

 

How did the air get in there? Is the brake pedal spongy? Air is a compressible gas and this would cause the pedal to feel spongy. I would be worry if their was air in the brake system for any reason. My $0. 02 I would be to go to a good brake shop-some one you trust. I would have the brake system flushed and replace with new brake fluid.

I do not mean to alarm you but I just do not understand how air was in the brake system. I do not think the factory would ship the truck this way. Being that this is a safety issue and this would leave them open to a law suit.

Good luck.

 

I understand your concern. The air they were referring to is in the hydro assist system, not the actual brakes. I was just looking at the plumbing for the power steering/hydroboost to see if I can maybe bleed it off somehow????

The brakes work the same, there is just a hiss in the boost system thats annoying.

 

Not an expert on this system by any means but I think you can dismiss the air bubble theory. There is a pressure relief valve located somewhere in the system which maintains a certain pressure on the fluid going to the hydroboost. I think that would be where the hiss is coming from. bg

 

That was my original thought. . I am not sure why that would just start hissing all of a sudden? Not sure what to do here... Try another tech I guess. .

Thanks

 

Recent developments.

Today I went to town on the system doing full lock to lock turns with the steering and holding the brakes to full travel and releasing quickly to get the flow and bypass to cycle.



The good news, its gone away, sort of. . Now if I tap the brake pedal slightly, it goes away. IF, I pull back on the pedal with my foot, it comes back. . So now I have narrowed it down to the booster, just why is it hissing when I pull the pedal fully back?



Input please... Thanks.

 

Listen at the hoses,any restriction or edge can create that type of hissing noise. I have fixed a few by replacing the hi pressure hose from the pump to hydroboost unit

 

The hoses have a teflon like liner inside them. The liner has come loose and is stuck in one of the fittings causing a slight restriction. If you pull the lines off and look closely in each fitting (both sides) you should be able to find it. When you step on the brakes or turn the wheel you are reducing the pressure within the system and the noise goes away.

 

So pulling the pedal fully back is opening the port for this piece being stuck in there? If I tap the brake I can get it to stop, sort of. It makes the hiss just like if you pump the brakes hard when you are parked.

The service writer wants to bring it in now that I described what is going on.

 

:-laf Exactly what I have seen I just don't want to give up all my inside info.

I have to work for a living :{

 

I appreciate the advice guys. Thanks a bunch. . I will book it in for a fellow technician to take a look at it. .

 

Follow up....

Firstly I want to give a huge thanks to Bob4x4 and Sag2 for their expert diagnosis from long distance. I understand sensitivity to keeping your trade secrets.



Being that I have never left something for someone else to fix, I got into investigating the tubing to the brake booster. After I pulled off the fitting and got the tubing out, it was pretty apparent what the problem was. The tubing was flared and split with signs of rubbing on the spool inside the booster.



The question is I just trimmed it, I didn't see an O ring fit to the tubing in there, will that be OK or do I need to replace the entire tubing elbow?



Thanks again!. .

 

Ahhh darn. . spoke too soon. After about a 10 minute drive, it started to come back again, only now a little less audible. Also I cannot get it to quit with just a tap of the pedal anymore. I looked in the port with a light and mirror and didn't see anything, tried to flush it out some too. The drain side I took out as well with no signs of debris.



Do you trim back the tubing to near flush with the fitting as it was on the drain side?



Here is what I found. .

 

I would replace the hose, but just try and trim it off if you don't want to spend the money. The trouble is the lining will probably just keep trying to move down the hose. It's worth a try though.

 

Thanks. .

 

Lil' Dog'

I would replace the entire hose assembly with a new one, if the liner is moving in the hose assembly.

The way a hose is constructed is in layers. The first layer is the inner liner which in this case is the teflon liner. This material is compatable with the brake fluid being used and will also resist the pressure of the fluid as it is carried in the hose. Then there is other material that is wrapped in layers around the inner liner to support the liner against pressure failure and to prevent the liner from being cut by abrasive rubbing. The last layer is the outer cover, this material is resistance to abrasive rubbing also and from oil and fuel spills that will deteriorate the teflon.

Once the integrity of the hose is compromised, the fluid being carry can cause internal failure. Either due to a pressure failure or to the outer layers not being compatable with the fluid being used. This will eat a way the outer material until the hose fails.

I know I have seen this many times on heavy equipment when the inner liner has failed. At least this is cheaper then a brake booster.

 

Is this a TSB yet? It should be if its that common. I will see what I can do about the hose, should be warranty.

Jim, thanks. I was initially surprised by the lines being of the high pressure design like a hydraulic hose, unlike typical PS hoses. But to have a liner it in it just for ATF seems odd to me.

 

Another update.

Just for a test I trimmed the tubing off only leaving about 1/4 to 1/2" protruding from the steel fitting and re-installed. The hiss was gone then slowly came back until today. I pulled the hose again and found the exact same thing. The tubing is working its way out and the hose replacement sounds like the only answer. FYI



Here is a photo of todays tubing. . Hard to tell I actually trimmed it.

 

Replaced the hose and I now see that the Teflon liner was only supposed to be in the rubber part of the hose, not in the steel fitting to the booster. Thanks again for the diagnosis.