Soft Brakes when car is on...Hard when off.

[RedfireV8]

Note: You can skip to 3rd paragraph to get to the main question.

So I've been hoping to make my first thread back to the Mustang world be about the car I recently picked up and a nice little story and pics to go along with it but that's not the case with this new issue. I've actually have never encountered this before but am looking for help.

The car is an 07 GT with the FR500S big brake kit. Recently rebuilt the calipers with new seals and dust boots and some fresh paint. Reinstalled them and gave em a nice bleed. Just a quick bleed to get it on the road. Brakes felt great. This was about 2 weeks ago and have driven it a couple times with no issues what so ever. Now the brakes were fine and all but I wanted to go ahead and get all of the old brake fluid out so I went to bleed them again.

After bleeding them they were rock hard...but as soon as I turned the car on, the pedal goes straight to the floor. You can pump it some it get some of the pedal to come back but within a few seconds it goes right back to the floor. Turn the car back off and you have a rock solid pedal. I've never seen this before. A quick Google shows this on a few Integra and 3rd Gen sites stating it to be the brake booster. Could this be my problem? It's crazy how all of sudden this would happen with no prior issues. Just looking for a little guidance from you guys. Any help is appreciated.

 

[CPRsm]

Pumping and gaining firmness is a sign of air. When you pump and it compresses it becomes firm. The booster activating and pedal going down was the same thing. It's applying more force and making a small problem more noticeable.
Try pumping the pedal w the power brakes on, and hold the pedal. If the pedal slowly goes to the floor there are two possibilities. There is a leak or and internal bypass(leak) inside of the system. If a small external leak, that will put air in the system also.

 

[RedfireV8]

Just so I understand and please tell me if I'm wrong. The fact the brakes go down when the engine is on, means the brake booster is in fact working. Correct? If so there must be a leak somewhere. Am I understanding correctly?

Visually just went and checked all four corners. There is no leak at any of the calipers, whether it be the lines or bleeders. Also made sure there was no wetness at the ABS module or the master cylinder. No wetness or signs of leakage at both.

So with all of that and from what I have been reading, the MC can actually be at fault but show no external signs (leaking). Is this also correct? If so, could it be the MC?

Oh and thanks for the quick reply.

 

[Ryan331]

There could also be air in the ABS system which can contribute to your problem.. And correct, the soft pedal with the car on, means that the assist is working (it multiplies the pressure you apply to the brake pedal)

 

[RedfireV8]

Quick update: Did a quick test by remove the vacuum hose going to the brake booster and plugged it. With the engine on the brakes felt rock hard. Immediately plugging the hose back into the brake booster resulted in soft pedal again. Can we assume its the brake booster? Let me know and thanks again guys.

 

[Ryan331]

Did you bleed the brakes using vacuum or pump/hold/gravity?

 

[CPRsm]

Quick update: Did a quick test by remove the vacuum hose going to the brake booster and plugged it. With the engine on the brakes felt rock hard. Immediately plugging the hose back into the brake booster resulted in soft pedal again. Can we assume its the brake booster? Let me know and thanks again guys.

Yes, that's the booster working. The pedal going "to the floor" is a problem. The pedal will will go down farther when the car starts, even in a perfect system. That's normal.

 

[RedfireV8]

Bled them using a Motive Power Bleeder. So technically pump. At each side starting with the caliper furthest away. Thanks again for the quick reply guys.

CPR:I did happen to stumble upon this thread. http://www.s197forum.com/forum/showthread.php?t=103876&highlight=soft+brakes But according to SoundGuyDave the fact that I plugged the vaccuum line means the BB is the culprit but according to you that's not the case and the BB is working fine. For whatever reason, I'm having a hard time wrapping my head around this.

 

[Ryan331]

It's possible it could be the brake booster, could also be master cylinder or could still be air in the lines /ABS system.. My concern is that this only happened after bleeding and was fine before..

Do the brakes actually apply when the pedal goes to the floor?

 

[RedfireV8]

It's possible it could be the brake booster, could also be master cylinder or could still be air in the lines /ABS system.. My concern is that this only happened after bleeding and was fine before..

Do the brakes actually apply when the pedal goes to the floor?

Just went and did a test drive. Yes, the brakes would start catching towards the end of the travel, right before hitting the floor.

 

[RedfireV8]

I should also add this as a note which I should have mentioned earlier. My apologies. As we know on 4+ piston brakes there are 2 bleeder screws per caliper. On my drivers side, the inner bleeder screw will not budge. No matter how much "careful" force I put on it, it will not move. I decided to just bleed the outboard side figuring they are connected being that the outboard pistons receives the same fluid that the inboard side does. I could be wrong in thinking that way though. Let me know if this is wrong. Again my apologies for not bringing this up sooner.

The more I think about it though, I really don't believe this would be the issue being that the brakes are rock hard with the car off and soft with it on. Furthermore with the brake booster line plugged and the car on, they are also rock hard. I would think with air in the lines they would be soft both on and off. Again could be wrong and looking forward to any further help.

 

[CPRsm]

Jess, both sides. Try a penetrant first

 

[RedfireV8]

Tried and it will not budge. It almost seems as if it was welded shut or something.

 

[Vorshlag-Fair]

As I posted in another thread, we have had TWO of the brake booster check valves fail (one in my 2013 and one in my 2011). We'd have perfect brakes except after a long straight (with a lot of WOT) and then boom, no assist. See the image and part number above. That's a $5 part and now we keep a new spare in the trailer. That will cause a rock hard pedal, with no brake assist. All the vacuum boost leaks out. Doesn't sound like your problem, but it is worth mentioning again.

The other brake-related issue I mentioned in the same thread was a bad master cylinder - which DID act like your car, in a way. But in that case I had burned up the pads completely, then boiled the brakes so badly that I lost pedal and drove off track. We replaced the pads, bled the fluid and... no pedal. Mush, mush, mush. No matter what we did it wouldn't firm up. Replacing the MC (about $85) fixed that, so an internal seal had failed... but right after an incident where I boiled the fluid in a big way.

Your situation sounds like the system is just not bled all the way. The booster working (starting the engine) seems to amplify the issue, as others have said.

The stuck bleeder in the caliper needs to be fixed, for sure. They can and do fail, but they usually fail by leaking (weeping) at the caliper and replacing the bleeder fixes it. A stuck bleeder is another story, and likely hiding the trapped air bubble in the caliper. I'd get it out by any means necessary. It might require a heli-coil repair if it tears up the aluminum threads in the caliper. Assume it will - so have that repair kit ready, along with a new bleeder. Then re-bleed the system.

How did it get stuck? Is the "hex" rounded off? Do you just drive in an ice/salt environment? Everything made of metal gets stuck or welded together on a car in the NorthEast. I don't know how you guys live with that situation... If a crusty NE car shows up here in Texas, shops won't work on them, they are unsellable, etc. You couldn't give me a NE car.

edit: see you live in Louisiana... next to the Gulf, perhaps?

Good luck,

 

[RedfireV8]

Thanks for all that info Vorshlag. Much appreciated and very helpful. I will check into the check valve being that its a cheap fix. I've never had good luck with drilling and tapping so I can only assume I'll have even worse luck. I've done 2 of em where both didn't come out good. One leaked and the other was destroyed when it was drilled out. Of course those were relatively cheap to replace.

In the mean time and I'll probably post a WTB, but does anyone know where I could find just the single drivers side caliper if worst comes to worst? I'll probably need some help from the vendors to find this. Ebay isn't coming up with too many options. Again guys I definitely appreciate all the help.

 

[86GT351]

Some basic questions here. Any visual leaks? Are you 250% sure the calipers are on the correct sides? Remember that the Bleeder screw needs to be on the Top. I ask because I have seen it before.

Another simple test can be done if you have an adjstable brake clamp tool. You can go ahead and clamp off all 4 wheels. With the car running if the pedal goes to the floor, your Master Cylinder is bad and bypassing internally. If it is hard, one wheel at a time remove the clamp and feel thr pedal. If there is a severe drop then it pretty much tells you which wheel has the issue

 

[RocketcarX]

Stupid question, is the stock master cylinder moving enough volume to keep up with that brake kit? Are the rears stock?

 

[K_Brogoitti]

The stock master cylinder has enough volume and output to put up with 4 piston brembos from the factory without a problem. If the pedal is going to the floor with the engine on most likely it is an improperly bled system or a failed master cylinder. Heat works wonders for removing stuck/seized bleeders. DO NOT use ANY petroleum based products to try and loosen the bleeder. If even a small amount of petroleum makes it into the brake fluid you could be looking at major problems from a contaminated system later on and the associated repairs. ($1000.00 plus).I use a small butyl torch to heat up the bleeder screw just enough to get it to break free. Be careful not to strip it. Try a complete re bleed after you get that bleeder screw popped loose. If that doesn't work the master cylinder is probably toast. The booster amplifies the hydraulic pressure enough to allow fluid to bypass the primary or secondary piston cups and seals that are possibly damaged in the master cyl. Pumping the pedal with the car off won't build enough pressure for the problem to come up.

 

[RocketcarX]

The stock master cylinder has enough volume and output to put up with 4 piston brembos from the factory without a problem. If the pedal is going to the floor with the engine on most likely it is an improperly bled system or a failed master cylinder. Heat works wonders for removing stuck/seized bleeders. DO NOT use ANY petroleum based products to try and loosen the bleeder. If even a small amount of petroleum makes it into the brake fluid you could be looking at major problems from a contaminated system later on and the associated repairs. ($1000.00 plus).I use a small butyl torch to heat up the bleeder screw just enough to get it to break free. Be careful not to strip it. Try a complete re bleed after you get that bleeder screw popped loose. If that doesn't work the master cylinder is probably toast. The booster amplifies the hydraulic pressure enough to allow fluid to bypass the primary or secondary piston cups and seals that are possibly damaged in the master cyl. Pumping the pedal with the car off won't build enough pressure for the problem to come up.

torch to a brake caliper? Seems legit. What do you do with the dot 3 fluid in it that he cooks with the torch? Also, I'm curious how you could contaminate the entire brake system (or much less any part of it) by using penetrating oil on a hole that serves exclusively as an out hole? If need be, remove the caliper and drill out the bleeder, it's standard practice in the brake service world and the debris is easily enough removed with air and the following bleeding.
If the master is bypassing, it will do it with the car off, exercise patience, press a little harder and wait. If it's a booster issue, it will only show itself with the car running. super simple.

 

[RedfireV8]

Some basic questions here. Any visual leaks? Are you 250% sure the calipers are on the correct sides? Remember that the Bleeder screw needs to be on the Top. I ask because I have seen it before.

Another simple test can be done if you have an adjstable brake clamp tool. You can go ahead and clamp off all 4 wheels. With the car running if the pedal goes to the floor, your Master Cylinder is bad and bypassing internally. If it is hard, one wheel at a time remove the clamp and feel thr pedal. If there is a severe drop then it pretty much tells you which wheel has the issue

Stupid question, is the stock master cylinder moving enough volume to keep up with that brake kit? Are the rears stock?

No stupid questions here fellas. I appreciate everyone's input. No visual leaks. I have checked this multiple times including tracing each line to the ABS and to the MC. And yes the bleeders are pointing up. That's a good idea about clamping off each caliper, unfortunately I only have 2 clamps and that would only work on the rears. Oh and the rears are stock. I would need to purchase 2 "quad pad" spreaders to complete the test. Very good idea though sir. Something to look more into if I start running out of ideas.

The stock master cylinder has enough volume and output to put up with 4 piston brembos from the factory without a problem. If the pedal is going to the floor with the engine on most likely it is an improperly bled system or a failed master cylinder. Heat works wonders for removing stuck/seized bleeders. DO NOT use ANY petroleum based products to try and loosen the bleeder. If even a small amount of petroleum makes it into the brake fluid you could be looking at major problems from a contaminated system later on and the associated repairs. ($1000.00 plus).I use a small butyl torch to heat up the bleeder screw just enough to get it to break free. Be careful not to strip it. Try a complete re bleed after you get that bleeder screw popped loose. If that doesn't work the master cylinder is probably toast. The booster amplifies the hydraulic pressure enough to allow fluid to bypass the primary or secondary piston cups and seals that are possibly damaged in the master cyl. Pumping the pedal with the car off won't build enough pressure for the problem to come up.

torch to a brake caliper? Seems legit. What do you do with the dot 3 fluid in it that he cooks with the torch? Also, I'm curious how you could contaminate the entire brake system (or much less any part of it) by using penetrating oil on a hole that serves exclusively as an out hole? If need be, remove the caliper and drill out the bleeder, it's standard practice in the brake service world and the debris is easily enough removed with air and the following bleeding.
If the master is bypassing, it will do it with the car off, exercise patience, press a little harder and wait. If it's a booster issue, it will only show itself with the car running. super simple.

Used a torch a couple times on hard-headed O2 sensors. Another good idea I hadn't thought about but like mentioned, generally bleeders are made of "soft" metal so I'm sure the heat may just make things worse including what was mentioned about the fluid. Which btw just in case the question comes up, I'm using DOT4 fluid. Anywho I definitely appreciate all the help and ideas fella's.

So what I'm getting from all my research on this forum and others (Google searching), it's the brake booster. I can replace the check valve as that's a simple and cheap fix. Air in the lines could also be it as I can't bleed the right side of the left caliper.

Again thank you to all for your generous help. Please keep it coming!