When brakes are applied hard, dead pedal for a sec.

[mhuppertz]

1989 Reatta

ABS light on all the time.

If I slam on the brakes, the pedal feels dead for maybe two seconds, then the brakes come on full blast.

What is the most likely cause of this. bad accumulator?

[Barney Eaton]

Lets think it out.......If the accumulator is good, there should be immediate pressure available for the power assist.

(It's a little complicated but basically the pump, pressure regulator, accumulator are for the power assist part of your brakes)

If you have a hard pedal, it is usually because you are getting NO power assist.

You may have a condition where there is little pressure in reserve, but when you press the pedal, it wakes up the system (pressure switch turn on the relay that turns on the pump)

once the pump is running you then get pressure.

The system is designed to operate between roughly 1500 and 2500 psi....you probably get some assist below 1500 but it is only enough for "normal" stops.

When you "panic" stop, you are demanding more from the assist, so if it is low, it takes a few seconds for the pump to supply the needed pressure.

Without having a pressure gage attached to the system to read the pressures, the first and easiest test is the fluid level drop in the reservoir. This give you a clue of the accumulator condition.

Unplug the pressure switch connection and inspect it....if it looks damp, you probably have brake fluid leaking thru the seals in the switch...this will throw off the calibration and it will fail.

[Barney Eaton]

Ronnie......we may need to alter the brake test.

This statement......If the fluid level change is 1/2 inch or less, your accumulator is on its last legs.

 

Should be 1/2 or more.....

This is based on using the OEM type accumulator.... as more Reatta owners find alternate sources for accumulators, their volume and precharge may be different which will affect the 1/2measurement..

[Ronnie]

Barney, I'm trying to wrap my head around what you are saying. " Should be 1/2 or more..... " I'm sure you are probably right but it just isn't soaking in for me.

 

The statement......If the fluid level change is 1/2 inch or less, your accumulator is on its last legs. ... to me is saying: "If the fluid level in the reservoir does not drop at least 1/2" when the pump fills the accumulator, then the accumulator is bad.(already full of fluid)" It would seem that would still be true if a slightly larger accumulator was installed because the fluid level in the reservoir would drop even more if you had to fill a larger accumulator..

 

Would it be clearer if it said: "The fluid level in the reservoir should drop 1/2" or more. If it doesn't your accumulator may be bad. or getting weak"?

 

I will be happy to change it to whatever wording makes the most sense. Just let me  know how you think it should be worded.

 

Along these same lines, I have been thinking of including a bench test procedure to help determine if an accumulator is bad by inserting a thin blunt ended rod into the hole in the accumulator to see if the bladder can be felt and/or pushed back. I did the test on my old accumulator that is known to bad and found that the rod goes deeply into it without resistance. On my good accumulator the rod stops just a short distance inside the hole when it touches the pressurized bladder. Do you think that would be a useful test to add to the tutorial?

 

[Barney Eaton]

I don't like the rod test because it only shows you really bad accumulators.......think about a 700 psi precharge on a good one, none of us could budge that with a rod.

But someone might aggressive and try real hard to compress the diaphragm and puncture a good accumulator.   I would think if you can move the diaphragm on a accumulator, it is very bad.

 

Obviously the accumulator will store/allow some amount of fluid in as the pressure rises......as the PRECHARGE diminishes, the same pump pressure will push MORE fluid into the accumulator.

This MORE amount of fluid will show up as a DROP in the level in the accumulator.

LESS drop is GOOD

MORE drop is BAD

With a OEM accumulator, 7/16 -1/2 inch is where I start to get concerned with the condition of the accumulator.....however I have seen some with 9/16 drop still working fine.

Call me it this is still unclear....512-869-5114

[Jim]

Back to the original problem.

If the red "brake" light is going out that means the accumulator is pressurizing and has the 2600 pounds of pressure in it to give at least one application of power assist immediately. A shot accumulator might not have the reserve pressure for a second application of the brakes but will on the first.

The problem with these systems is the valve body valves getting corroded because brake fluid naturally attracts moisture and this moisture can corrode the valves and cause a hard pedal and no brakes and/or on a hard application of the brakes the car will pull to one side or the  other.

I bought one parts car years ago because the fellow knew he had some sort of a problem and then a truck pulling a trailer pulled out in front of him, he hit the brakes and hard pedal and no brakes. Totaled the Reatta.

A few years ago my wifes '90 was getting so bad even a slight hard stop would pull her car to the right. I changed the master cylinder/valve body and no problems since.

This is why it is so important to change the brake fluid every 5 years to get the moisture out of the fluid before corrosion can start.

My guess in this situation the valve body needs to be changed or the whole unit on the firewall.

I feel the car is not safe to drive in this condition as you never know when you might have to make a panic stop.

On these same lines, I have had opportunities to purchase complete units but pass on all of them unless I can personally drive the car and perform a test panic stop to make sure the car stops correctly and does not pull to either side.

[Ronnie]

1 hour ago, Barney Eaton said:

This MORE amount of fluid will show up as a DROP in the level in the accumulator.

LESS drop is GOOD

MORE drop is BAD

 

I can understand that to a point but I don't think it can be used as a rule of thumb for determining if an accumulator is completely bad. Weak maybe.

 

Here is an example of why I say that: - Last year I talked with an 80 year old Reatta owner at a small car show in Spring City, TN. He told me both his brake warning lights would "flicker" when he first hit his brakes but the car was stopping fine. He told me he had talked with a Reatta parts vendor that had diagnosed the problem over the phone (not Jim) and was told he needed a rear ABS wheel sensor lead. He ordered the wheel sensor but hadn't yet installed it. He asked me if I could help.

 

When I looked under the hood I noticed the fluid level in the reservoir was right on the full mark. I thought that was odd since he had driven the car to the show only about 30 minutes earlier. I thought the level should have been down some at that point.

 

I asked him to pump the brakes 25 times and the fluid level moved up slightly but stayed basically the same. When he turned the key on and the pump ran, the fluid level stayed on the full mark and the lights went out. I knew that wasn't right.

 

After some thought I came to a logical reason for that. - The bladder in the accumulator had ruptured and the accumulator had completely filled with fluid on both sides of the bladder. All the nitrogen had been pushed out over time and replaced by fluid. Since fluid won't compress there was no pressure stored in the accumulator to push the fluid out when he pumped the pedal 25 times. When he turned the key on, causing the pump to start, it only ran a couple of seconds and cut off because the accumulator was already full.

 

The next week I went to his house and installed a good used accumulator I had on hand and his brake system started working normally again with no flickering warning lights. The fluid level dropped as it should. He was very happy about getting his car fixed. He gave me the wheel sensor lead he had ordered so we were both happy.

 

We emptied the contents of the accumulator into a Dixie cup. As far as we could tell the accumulator was completely full of fluid. That  is when I got the idea to stick a coat hanger into the hole in the accumulator to see if I could fee the bladder. There was no resistance at all until it bottomed out. The bladder must have been pushed up against the dome of the accumulator.

 

At any rate, I want the accumulator test tutorial to be as easy and accurate as we can make it. Just let me know what it should say and I will make the changes on ROJ.

[2seater]

If the reservoir could be calibrated in some manner, it might provide a valid measure of the accumulator capacity. I know we have an approximate fluid drop standard, but I do not know how regular the interior shape of the reservoir is to fine tune the actual volume change. I know the accumulator has a capacity listed but not at what final pressure that it is measured at. I think I have an operational pump and motor and although it is taken apart at this time, I would believe it should work to make a test rig. I do not have the motor and switch harness but I should be able to power the motor directly and switch it manually while watching the gauge setup I made previously. I'm thinking my head cc'ing burette should work as a reservoir? I know this has been an issue that has vexed us for a long time.

[Digger914]

On ‎4‎/‎2‎/‎2017 at 8:42 PM, mhuppertz said:

1989 Reatta

ABS light on all the time.

If I slam on the brakes, the pedal feels dead for maybe two seconds, then the brakes come on full blast.

What is the most likely cause of this. bad accumulator?

 

A brake accumulator is like a battery and an accumulator with a busted bladder is like a nearly dead D cell, a meter might say it has a volt and a half inside, but it hasn't got enough power to brighten up a small light bulb. A bad accumulator will keep the brake fluid from hosing down the engine compartment when the pump runs up enough pressure  to turn out the brake lights, but that's about all it's good for, it doesn't hold enough charge to compress the calipers on a hard stop. As soon as it releases the little pressure it has, the pump will kick in to restore system pressure. That is until working the pump on every stop, eventually works the pump so much that it doesn't work anymore.

 

Flushing brake fluid doesn't have a set in stone interval and how often it should be done is more dependent on storage, climate and usage than it is on calendar years. Dot 3 brake fluid collects moisture, Dot 4 will too, but it takes a lot longer and when it comes to brake fluid, if in doubt, flush it out. Good fluid in the reservoir doesn't mean that it's good all the way down the line. One of the great bad habits of the people who work on disk brakes was once an approved service procedure and compressing the calipers without opening the bleeders causes a lot of ABS trouble.

 

A full time ABS light could be caused by a stuck relay, blown fuse, bad controller and even a bad pressure switch, but it's usually a bad wheel sensor. A sticking valve doesn't light the light on these older GM's, but you will know if you have one when you stomp on the peddle.