Brake Pedal Gradually Goes to the Floor

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
I'll give the full back story to this in order to try avoid wasting anyone's time with follow up questions.

1998 Discovery 1, with ABS.
The initiation of the issue was after I installed new rear rotors and pads. Once everything was back together I couldn't get the calipers to bleed. I tried both the two man bleed method and a MityVac hand pump, both to know success. I ended up buying a pressure bleeder so that I could stop calling my wife to the garage to push on the brake pedal. However, despite that building pressure, no fluid came through to the calipers, even the front ones. I finally tracked the issue to the master cylinder. The pedal bleeding had pushed old corrosion to the point of blocking the outlet and no fluid was getting out of the master cylinder.

So, I replaced the master cylinder with one from Atlantic British. From there the system bled normally (pressure bleeder) and I was able to get pressure back. However, upon driving the car the first time, the brakes became spongy. I checked for leaks and found oil on the passenger rear rotor. I tracked that down to a blown stub axle seal, but replaced the caliper pistons and seals for good measure, along with new pads. Then I bled the system again for good measure. Again, when I drove the car the brakes became spongy again. I drove it this way for a few months, just occasionally for local errands, as I didn't have time to further troubleshoot. Although the brakes didn't inspire confidence they were still functional.

Then a few weeks ago, I found a leak at the driver's front caliper. This was clearly from caliper the pistons and seals and so I replaced them, along with new pads. Again, I bled the entire system at each caliper. The pedal felt solid as a rock. However, as soon as I drove, it became spongy again. So, I spent today doing some additional testing.

1) Inspected for leaks at all hoses and calipers. Found none and fluid level in reservoir is not changing.
2) Inspected seals where the reservoir meets the master cylinder (even though they're less than a year old). No leaks
3) Sprayed the reservoir cap with soapy water to check for air leaks there, but nothing.
4) Checked the hose from the brake servo to engine for vacuum retention, on the engine side, all good.
5) Rebled the system and found no air in it.

Current Symptoms:
Pedal is solid with the engine off
Pumping the pedal doesn't increase it's firmness
With the engine on, applied pressure will result in the pedal gradually going the floor (over the course of a minute or more)

Any suggestions on where to look from there? Thanks in advance for the help.
 

squirt

Well-known member
Nov 13, 2008
824
13
Los Angeles
Are you certain that you don't have air in the ABS block? The thing that triggers this question for me is the fact that the pedal seems to be rock solid when ABS would be inactive.

Did you happen to bench bleed the new MC prior to installation? If not, it may have pushed a bunch of air into the system which could get hung up in the ABS valves. I know you've bled the system about 10 times by now, but I'm guessing you're always doing it with the vehicle off, meaning the ABS is inactive.

If it was mine, I'd try to activate the ABS while bleeding (there are various ways, but I'd have it off the ground with the wheels turning) in an attempt to ensure there's no air stuck in the ABS block. I think this is the source of your issue.
 

Tugela

Well-known member
May 21, 2007
4,754
561
Seattle
A shop can also do a Testbook bleed, which will purge the air from the ABS block and MC. Might be worth the cost depending on your patience and value of time and money.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Are you certain that you don't have air in the ABS block? The thing that triggers this question for me is the fact that the pedal seems to be rock solid when ABS would be inactive.

Did you happen to bench bleed the new MC prior to installation? If not, it may have pushed a bunch of air into the system which could get hung up in the ABS valves. I know you've bled the system about 10 times by now, but I'm guessing you're always doing it with the vehicle off, meaning the ABS is inactive.

If it was mine, I'd try to activate the ABS while bleeding (there are various ways, but I'd have it off the ground with the wheels turning) in an attempt to ensure there's no air stuck in the ABS block. I think this is the source of your issue.

I thought about the air issue in the ABS block. I did bench bleed the new MC. However, prior to identifying the issue with the previous MC, some air was more than likely pulled into the system, as I was using a vacuum pump on the calipers and no fluid was coming out of the bad MC.

The symptoms I'm experiencing don't seem to indicate air in the lines, i.e. pedal descending vs. firming up when pumped. However, I don't know if those same symptom parameters extend to air in the ABS modulator. I had spoken with a tech at Atlantic British who advised that if I was using a pressure bleeder and set the pressure north of 11psi, it would force open the solenoids in the ABS and serve to bleed the unit. So, when I've done my bleeds, I've had it around 12psi. However, you are correct that I've always bled with the engine off per the Rave instructions.

I know Rave indicates the servo or MC as issues with this particular symptom. I had been thinking the servo since the symptom presents with the car on. However, it has vacuum and clearly provides assist. I pulled the new MC from the servo and double checked it for leaks, just to be sure it wasn't a bum one. Another forum suggested the brake pressure reducing valve as the issue?

Nevertheless, I'd be happy if it was simply air in the ABS rather than parts. Does the advice regarding the pressure bleeder being able to activate the ABS ring true? I have a scan tool, but I've never checked to see if it has the feature to activate the ABS. I'll take a look at that when I get home and investigate alternate methods for bleeding the ABS further. Any step by steps would be appreciated.

Thanks for the help guys.
 

SGaynor

Well-known member
Dec 6, 2006
7,148
162
52
Bristol, TN
Nevertheless, I'd be happy if it was simply air in the ABS rather than parts. Does the advice regarding the pressure bleeder being able to activate the ABS ring true? I have a scan tool, but I've never checked to see if it has the feature to activate the ABS. I'll take a look at that when I get home and investigate alternate methods for bleeding the ABS further. Any step by steps would be appreciated.

Thanks for the help guys.

Pretty sure it won't.

You don't need a computer to bleed the ABS. Find a big parking lot. Do lots of starts/stops that engage the ABS. That'll get the air moving.

Re-bleed the brakes.

Also check the rubber parts of the brake lines - if they are expanding under pressure that will cause the brakes to "give." If they are original (20 years!), the rubber might be dying/dead.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Thanks. I'm not sure the ABS will engage with the current power the brakes are offering, but I'll give it a shot. Should the follow up bleed be done with the car running then?

I've checked the rubber hoses and they show no signs of a leak. I figured expansion in those would account for a soft pedal, but they'd have to leaking for the pedal to go to the floor. However, given the age of the car I was considering replacing with stainless steel anyway.
 

Rob371

Well-known member
Nov 29, 2016
150
1
Charlevoix, Michigan
Firstly bench bleed master cylinder. Did you bench bleed the new master?

Second step is power bleed abs. Drive around in parking lot like a tard and slam on brakes a bunch. Then bleed manually. Waste of time but it was fun right? Now take to shop with ability to scan and power bleed then have shop perform another manual bleed.

For the money you spend at a shop on this one repair you could be half way or better to owning a scan tool that does all that and more.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Firstly bench bleed master cylinder. Did you bench bleed the new master?

Second step is power bleed abs. Drive around in parking lot like a tard and slam on brakes a bunch. Then bleed manually. Waste of time but it was fun right? Now take to shop with ability to scan and power bleed then have shop perform another manual bleed.

For the money you spend at a shop on this one repair you could be half way or better to owning a scan tool that does all that and more.

I guess some of this was buried in my long responses and overlooked. I have both a pressure bleeder and a vacuum bleeder as well as a scan tool. I just need to double check if it has the capabilit to cycle the ABS.

I mentioned that the new MC was bench bled, but that air was most likely pulled into the system by my vacuum bleeder before I realized no fluid was coming out of the master.

Unfortunately, my experiences with three local shops to this point have been complete wastes of time and money. Happy to relay my overheating saga the last time I took the car to a shop. Anyway, if someone knows of a good one in the San Fernando Valley, I'd love to know of them.
 

SGaynor

Well-known member
Dec 6, 2006
7,148
162
52
Bristol, TN
Second step is power bleed abs. Drive around in parking lot like a tard and slam on brakes a bunch. Then bleed manually. Waste of time but it was fun right? Now take to shop with ability to scan and power bleed then have shop perform another manual bleed. .
You do know that using a computer to open/close the valves in an ABS unit is exactly the same thing that happens when you slam on the brakes and engage the ABS, right?

Thinking that using the computer is the only way to bleed the abs is wrong.

Oh, and if there is air in the ABS valves, using the brakes normally won't cause the pedal to go to the floor - that trapped air is, well, trapped, and not part of the brake system. That is until you engage the ABS.
 

Rob371

Well-known member
Nov 29, 2016
150
1
Charlevoix, Michigan
You do know that using a computer to open/close the valves in an ABS unit is exactly the same thing that happens when you slam on the brakes and engage the ABS, right?

Thinking that using the computer is the only way to bleed the abs is wrong.

Oh, and if there is air in the ABS valves, using the brakes normally won't cause the pedal to go to the floor - that trapped air is, well, trapped, and not part of the brake system. That is until you engage the ABS.

To be fair, I considered this method myself when I replaced my ABS module. I understand, you do what you can with what you have.

I am fortunate to have a snap on scanner so for an extra 120 dollars I bought the special key chip and data cable that allows communication and test functions with all modules. Using the scanner, and a stick for holding the pedal down, I was able to bleed the system in about twenty to thirty minutes without issue and I didn't have to crawl under there to manually bleed it a second time.

I see scan tools running from 400 to 800 bucks that will do everything on a land rover. It's easy to spend 400 to 800 bucks on one trip to a repair shop.

When most everyone seems to question the competence of their local repair guys, which I understand wholeheartedly, a scanner and the ability to do it yourself seems like a worthwhile investment.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Are you sure Vacuum brake servo diaphragm is ok? No vacuum losses?
Regards

I pulled a vacuum on the engine side of the hose from the servo. It didn't produce a bunch of pressure, but held solid with no loss for 5 minutes. I also ran the car before pulling the new MC to check for leaks and there was an audible hiss when separating it from the servo.

My understanding is that the servo works off of multiple chambers though. So, I don't know that those two tests alone confirm that it's completely functional and not a cause of the issue.
 

Rob371

Well-known member
Nov 29, 2016
150
1
Charlevoix, Michigan
I don't mean to beat this over and over but a gradually sinking pedal in my experience is always associated with a failing master cylinder or air in the master cylinder. If there is no fluid loss any where then there's no other option. Perhaps ABS could be leaking internally. Mine did this and fluid was leaking from valve cover on top of module so the problem was clear with visual inspection but no other faults were present. Firm pedal, no fault lights,,... I just kept topping off the reservoir until I could get it replaced.

I have experienced new (remanufactured) masters leaking internally between front and rear. New doesn't always mean good.

Are you having fluid loss? They can leak at rear of master which ends up through the bulkhead behind the carpet. The booster is there to reduce the amount of effort required by the driver. If booster fails then you would experience excessive effort required to depress the brake pedal. A sinking pedal is a hydraulic problem, not vacuum assist.

Hope you get it sorted out.
 

mearstrae

Well-known member
Mar 15, 2017
143
18
Pennsylvania
Try using a vacuum gauge on the engine side of the brake booster hose to see what reading you get. Then use your vacuum pump (the one you use for brake bleeding) to put a vacuum on the booster and see if they hold about 20 inches of vacuum. Having a hiss when you pull the hose is a good sign though. As said, having an internal leak in the master cyl. or the ABS unit is a real possibility.

'99 Disco II
'95 R.R.C. Lwb (Gone...)
'76 Series III Hybrid 109
'70 Rover 3500S
 

discostew

Well-known member
Sep 14, 2010
7,706
1,015
Northern Illinois
I think most cars brake pedal slowly sinks to the floor if you stand on it hard and wait what was it you said, a min or 2? Sometimes after you fix a shitty brake pedal your over critical of it. If the truck is in drive how hard are you holding the brake pedal to keep the truck stopped? I'm guessing not as hard as if your sitting in park trying to push the pedal to the floor.

Compare it to another one?
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
I don't mean to beat this over and over but a gradually sinking pedal in my experience is always associated with a failing master cylinder or air in the master cylinder. If there is no fluid loss any where then there's no other option. Perhaps ABS could be leaking internally. Mine did this and fluid was leaking from valve cover on top of module so the problem was clear with visual inspection but no other faults were present. Firm pedal, no fault lights,,... I just kept topping off the reservoir until I could get it replaced.

I have experienced new (remanufactured) masters leaking internally between front and rear. New doesn't always mean good.

Are you having fluid loss? They can leak at rear of master which ends up through the bulkhead behind the carpet. The booster is there to reduce the amount of effort required by the driver. If booster fails then you would experience excessive effort required to depress the brake pedal. A sinking pedal is a hydraulic problem, not vacuum assist.

I believe I've officially ruled out the MC. I'll post details on that shortly. I'm not experiencing fluid loss. I only brought up the servo as Rave's troubleshooting section for this assigns a sinking brake pedal issue, without visible leaks, to either the MC or the Servo.

You may be right about the ABS leaking internally. More on this as well shortly.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Try using a vacuum gauge on the engine side of the brake booster hose to see what reading you get. Then use your vacuum pump (the one you use for brake bleeding) to put a vacuum on the booster and see if they hold about 20 inches of vacuum. Having a hiss when you pull the hose is a good sign though. As said, having an internal leak in the master cyl. or the ABS unit is a real possibility.

Thanks, I had already pulled a vacuum on the hose at the engine side. It was mentioned earlier in the thread though. The hiss was actually just a follow up confirmation of that reading. I believe I've ruled out the internal leak on the MC and you may be right on the ABS. More on both shortly.

Thanks for the help.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
Thinking a little more I agree, it's an hidraulic issue. Is this Disco equipped with a ANR3194 valvle or similar? Sorry i'm still using a non ABS 98D1
These regulator valves when dirty make trouble for bleeding

Yes it does have the ANR3194 valve. That was mentioned in another thread as a possible issue. It's a little down the list of probable causes though as the symptoms don't match up. When they go bad, my understanding is it's typically accompanied by either lockup at the rear wheels under braking or inability to bleed the rear calipers. I don't have either issue and there is no leak visible from it, which would cause my symptom. However, if all other diagnostics fail, I plan to isolate the system downstream of that valve and test to see if the sinking pedal presents or not.
 

ecaii

Active member
Feb 7, 2006
40
0
Van Nuys, CA
I think most cars brake pedal slowly sinks to the floor if you stand on it hard and wait what was it you said, a min or 2? Sometimes after you fix a shitty brake pedal your over critical of it. If the truck is in drive how hard are you holding the brake pedal to keep the truck stopped? I'm guessing not as hard as if your sitting in park trying to push the pedal to the floor.

Compare it to another one?

I've thought the same thing when driving it on flat ground and begun to question if there is an issue or it's in my head. However, if I have the car stopped on a decline, I'm praying for traffic to move before I creep into the car in front of me. In that case I need to continually apply more and more pressure in order to keep the car still, as the pedal continues to sink. So, something is there.