1970s Possable issue with hydraulic booster

[Steve56]

Started my 78 Eldorado this morning. Had no power steering and brakes were hard to press. Restarted motor and everything seems to be working now? No leaks or loose belts.  Any ideas will be appreciated.

[Julien Abrahams]

I don't think that the power steering and power brakes are related to each other as fas as I know. Was it running OK or did it die?

[Big Fins]

They are related if the car has the Hydro-Boost system. I never offered a reply because I don't know if the '78 model year used that or a vacuum booster.

[TJ Hopland]

76-78 Eldos do have hydro boost brakes so yes they are related.

After it started working did you check the fluid again?  Is the level still good?  Any signs of air bubbles in the fluid?   I'm wondering if you have something aging and some air is getting somewhere so its kinda got to bleed/burp itself to start working again?

The high pressure output from the power steering pump goes up to the brake booster.  Booster has 2 outputs, a high pressure to the steering box and a low pressure back to the pump.  Steering box has its own low pressure return back to the pump.

There must be some magic valving going on in the booster that distributes and or prioritizes the pressure between the booster and steering.  The booster also has an integral reservoir and check valve that just like the vacuum boosters stores a 'charge' that is intended to give you a few normal power stops without the engine running.   There were 2 different styles one I think used a mechanical spring and the other used a high pressure gas canister to accumulate the charge.  The gas ones its a visible gold canister sticking out of the assembly. 

You could have valving that is just sticking due to age usage, I know there are some 'soft' parts in these units.  I don't know if any of them could be causing this issue.  Not all the working parts of these use soft parts for sealing, I believe some components are just precision machined so they don't require any sort of seals.  Things could just be getting stuck or bypassing till the pressure comes up and either seals or unsticks?

I suppose another possibility is that its related to that accumulator part of the system.  Maybe the check valve is letting the system bleed down faster than it has in the past and it just needs time to 'recharge' and that recharge is literally sucking up all the pressure and fluid for a period of time leaving none for power braking or steering so this could be the new normal till the system is repaired.  Note that this may mean you will no longer have any reserve so if the engine stops so do the power brakes.   Maybe the spring or gas cylinder just failed and that start up had to push fluid where it normally doesn't go?  Again may be leaving you with no reserve.

This system was something I believe Bendix came up with in the late 60's early 70's for heavier light duty applications.  The kind of thing where air brakes would be to complicated and overkill but still needs a little more than you can get with the traditional vacuum booster.  They also are the ideal choice if you have something with no vacuum source like say a diesel.  Not sure why Cadillac thought the Eldo's needed it.  There were other GM rear disc cars in the era that didn't get it.

The system isn't rare or unique to these cars.  It was and is still commonly used on the heavier light trucks like 3/4 2500 and 1 ton 3500 trucks.  I'm not real up on the current trucks but my dads 2013 2500 gas Silverado has it and it more or less looks like the ones in the Eldo's we are talking about here.  I doubt the truck one would bolt into an eldo but I don't think the concepts or many of the internals are much different.

Several years ago when I did have to have one rebuilt I remember the shop saying that there was basically 2 rebuild kits that covered 90% of the units from the 70's into the 90's (which was what I was working on at the time).  Mine have been working so I have not had to deal with them at all the last few years so I don't know if anything has changed like Bendix or who ever owns them now discontinuing support or some large aftermarket discontinuing their support.  That sort of thing is kinda becoming expected these days,  2 cent part failed so you need to throw the whole (car in this case) away and buy a new one.