1976 Eldorado - Sticky Brakes.

[Eric DeVirgilis CLC# 8621]

After replacing all four brake calipers, brake hoses and bleeding the system numerous times, the brakes continue stick on - but only after the car is driven for 20 minutes or so. Each time the system was worked on the brakes would work fine initially, after which the brakes will eventually remain applied overheating the fluid resulting in a loss of pressure with each of the four wheels smoking terribly.

Does anybody have any idea whether the master cylinder or any other components of the brake system could be causing this?

*I should also mention the car has 17,000 miles and hadn't been driven in 30 years until recently. The master cylinder & booster assembly appear original to the car without any signs of having been tampered with. 

[Dave Shepherd]

A bad booster can cause this, though rare. Also check your pedal free play, should be about 3/8 inch.

[Eric DeVirgilis CLC# 8621]

A bad booster can cause this, though rare. Also check your pedal free play, should be about 3/8 inch.

Thanks for the reply. Can you elaborate on how the booster would cause this?

[TJ Hopland]

For those not familiar with the 76-78 Eldos note that these were a 4 wheel disc system that used a hydrovac booster rather than the typical vacuum booster.  Hydroboost was what was and is used on diesels and heavier light trucks.

The rear calipers are pretty over complicated for their own good due to the parking brake mechanism so having issues with those isn't unheard of.  Fronts are pretty standard except hoses are prone to issues but its said those are replaced so it seems unlikely the fronts would still be having issues.

That kinda leaves us with things that would effect all 4 which is the prop valve, MC, booster. I do wonder if maybe something is wrong with the match and fit between the MC and booster and its not quite letting the MC fully return to the rest position. 

I recall hearing about issues with the pushrod on the outlet of the booster being incorrect or not coming with the replacement booster and creating new issues when people try to swap them assuming they even notice they are different.   

[wheikkila]

I would start with the master cylinder. You say the car was sitting for 20 years. If the valve in the MC is not returning properly it will build pressure. You have only done part of the brakes. Now you need to complete the rest of the brake system. Forget about the 17,000 miles. It is still a 40 year old car that has not been driven. You will need to go through the systems on the car to bring them back. Good luck working it out. At the end you should have a very nice car.
               Thanks Wayne     

[Cadman-iac]

 I have a question for anyone who might know. I've noticed that the Hydro-boost unit used on the Eldorado does not have the typical pressure accumulator on the side like the ones used in trucks.
 This was used to give the driver a chance to stop the vehicle in the event that the engine has died and there is no pressure from the pump.
 So in the case of the Eldorado Hydro-boost, how did they get any emergency pressure reserve to stop the car?
 I have a Hydro-boost that I removed from a 78 Eldorado, and I've been trying to figure out what they were thinking about when they designed it without an accumulator.
 The only thing that I can think of is that because the Eldorado probably won't be towing any trailers, they figured that it would stop by leg muscles alone in the event of all power loss.
 I'd like to know, since I want to use one in my 56, and one with the accumulator will not fit where I want to use it.
 Anyone?

Rick

[Dave Shepherd]

TJ and Eric are on the money here. Careful here, but if you loosen the mc slightly  from the booster, this would eliminate  the issue of not enough  clearance between  the the mc and booster, causing brake drag when things heat up.

[TJ Hopland]

I missed the bit about sitting for 30 years.  I would not expect an MC that sat for 30 years to function properly.  Booster has 'soft' parts in it also that will likely sooner than later need some attention. 

The booster like used in the Eldo's still had an accumulator, it was just integrated into other internal components.

https://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/     

[Cadman-iac]

  I didn't know that there was a reservoir in the piston on these models of Hydro-boost. If that's the case, then I'd much rather use this one as opposed to the one I had planned on using. The one I was going to use has an external reservoir that is plumbed in with hard line, and it's the size of an A/C dryer, making it difficult to fit into the car.
Thanks for posting the link to that article TJ, much appreciated.

  Rick

Here's a picture of what I am trying to adapt to my 56. As you can see, this accumulator is huge compared to the typical ones used on later units. The Eldorado unit would be much easier to fit and look much better besides. There's not a lot of room in a 56 engine bay, and the typical Hydro-boost with the accumulator on the right side hits the power steering pump, thus the question about the Eldorado unit.

  Eric,
 My apologies for hijacking your thread here. I realized I should have started another after I posted my question.
 Rick

[Eric DeVirgilis CLC# 8621]

Sounds like the only way of covering all bases is to have the complete master cylinder/booster assembly rebuilt - if indeed one or the other is capable of malfunctioning in such a way to affect both axles. My mechanic, a thoroughly seasoned veteran of 50 years has never encountered a master cylinder that did this.  Either front axle or rear but never both because each is on a separate circuit.

Thanks for the responses.

[TJ Hopland]

If the piston in the master isn't able to fully retract for any reason like maybe a ring of corrosion from where it sat for 30 years that could totally cause your symptoms.  Maybe your mechanic hasn't worked on things that have sat for extended periods of time?

Looks like the master is available new from regular parts stores for about $100.  Not sure how many people rebuild them anymore or where you even get a rebuild kit.

Boosters look to be a little more tricky but what I was told is there are basically 2 rebuild kits that cover about 90% of units ever made.  You can check with diesel/truck shops that work on smaller (pickup sized) trucks, if they don't rebuild them they should know who does.   

[Eric DeVirgilis CLC# 8621]

He has a lot of experience with antique cars that have been sitting for longer than this. However, he probably saw none that sat this long that also have hydroboost.

What I can't understand is why the Fleetwood Brougham of the same year which also has four wheel disc brakes but is vacuum assisted and weighs even more than the Eldorado.

[TJ Hopland]

I never heard why the Eldo's got the hydro boost.  At the time you only saw them on things like RVs which you would assume was a weight thing and diesels which didn't have a good vacuum source.  Before the hydro boost the RV sorts of things had a sort of slave cylinder setup kinda like a clutch where the pedal system operated a slave cylinder that then operated the master cylinder.

It should not have been a space issue.  Eldo engine sat up higher than the RWD cars but it also sat a few inches to the right which would have likely gained back any clearance that was lost due to the height. 

If they thought it was a better feel then ya you would think why not the Fleetwood too. 

 

[V63]

I have had this issue before and it echos what has been suggested. Albeit not hydroboost.

Each time the brakes are applied the master cylinder is not returning fully and building increasing residual pressure each application.

Eventually The brakes will lock full on. To move car again you can loosen the lines at the master cylinder to release (bleed) pressure.

✅ There is an adjustment nut inside the booster / master cylinder connection. It needs to be shortened to allow the MC to return fully. While The master cylinder is off,  check signs of leakage on the back side.

Hydroboost has a very different 'feel' and I can see someone trying to modify that and tampering with that adjustment  which is very sensitive.

[Dave Shepherd]

I suggested loosening the mc also, had that issue on a Chev in the shop, that had an adjustable  rod out of the vacuum booster. Obviously  this is just a diagnostic procedure.

[V63]

I suggested loosening the mc also, had that issue on a Chev in the shop, that had an adjustable  rod out of the vacuum booster. Obviously  this is just a diagnostic procedure.

Yes, that would work too!!! at the time I was stuck on the roadside wondering what what going on with this car we just bought was to crack the lines. 🤦🏻�♂️✅