Low pedal (at times) '08 Sebring sedan
Good morning … still is lingering with this beauty. It has rear drum brakes
Initial complaint was: service writer noted low pedal to customer, which I am unsure if she actually noticed it. Repairs have gone thru 2 other techs over the last month or so, until my involvement. Customer also has a retired heavy duty tech boyfriend, who may be helping to be hyper critical of the pedal.
Regardless of the repairs already done, this is where we are, with little direction what to do next.
KOEO, pump pedal one time to empty booster, pedal is HIGH and HARD. No matter how hard you push. Start engine with foot on pedal, pedal easily drops a large amount. Very spongey. DOES NOT PUMP UP. AS SOON as it's placed in gear, you begin to feel resistance in the pedal in an acceptable amount travel. Driving pedal feels very good, but you can push the pedal lower, after you come to a full stop. But there isn't a feeling like you have to.
So this issue is the low pedal, engine running in park. Clamping off all hoses reveals that the rear brakes are not the issue. Releasing front calipers will result in the pedal dropping 1 ½ inch with each side. I removed both calipers and installed 1 inch steel blocks between the pads. Pedal is still low, engine run, in park. You can see the calipers ‘stretch’. Measured the opening: .010" movement w/o boost, .031" with boost. Piston appears to retract the normal amount. I went ahead a tried a new caliper on one side, expecting the pedal to get ½ better, if new calipers would fix it. There was no change in the pedal. Put old caliper back on.
We have reached out to the local dealer, who has put the VIN in their system, and no common issues have come up. Have tried ABS activations with no changes.
Rear shoes are adjusted and seem to operate correctly.
Assumptions made: Pedal firmness in gear is not due to rear brake rotation, as pedal acts the same on rack, with no rear wheel rotation. Rotor parallelism or caliper mounting is not an issue, proved by using separate blocks between the pads. Disconnecting the ABS module (electrically) changes nothing. Since pedal will not pump up (engine run in park) and KOEO high and hard pedal, I believe trapped air is not an issue.
We put on a factory master cylinder to be sure fluid volume is correct for the calipers.
Has anyone run into a similar issue? Thanks in advance.
Items replaced: 4 master cylinders (2 O'Reilly's reman, 1 NAPA new, 1 OEM), 2 used ABS hydro units, both front brake hoses. Gallons of brake fluid….well….it seems like it.
Hi Steve, FYI I changed this message from a Resource to a Question. You should have seen a warning note when you chose Resouce that says: “Note: Resources will only appear in search results and the Resources area. Resources do not appear in the homepage stream, nor do they generate notifications for most members.” The Resource type is mainly intended for uploading waveforms, scandata, and other…
Thanks Tom. I was so concentrated on getting all my ducks in a row, I missed that. Thanks for the fix.
Did you try bleeding the air directly from vehicle ABS pump and priming ABS pump with fresh brake fluid? Did you bleed the the brake calipers in correct sequence,pattern?
Before I answer, with all do respect, wouldn't air in the abs module cause a low pedal, despite the clamping of the front hoses?
Steve, This is on General Motors, but I always go through the bleeding procedure on the scan tool because small air bubbles do tend to collect in the ABS system. In most cases, pedal height is improved after using the scan tool bleeding procedure…
Thanks. No, I have not done an ABS bleed at this point. Reasons are 1, previous tech did a monstrous amount of ABS bleeding, on the first ABS and second master. And did not get as good a pedal as I did with old school bleeding. I cracked lines at the master with pedal being operated and also bumping up between cracks, cracking lines INTO the ABS with pedal actuation, cracking each line OUT of…
I would tend to think this is an overly critical situation. First off, free clearance needs to be taken up just to get the brake frictions to contact and begin to apply stopping action. Then there is a small amount of comprehensibility and flex of the friction material, shims and coatings. Based on some of the data you presented allow me to give you some perspective. Base on the boosted caliper…
Nice perspective. I presented the facts for this type of discussion. I do not think the flex is enough to be an issue. However, clamping off the calipers DOES fix the low pedal.
Steve, is there any end play in the front wheel bearings? A little end play goes a long way in creating extra pedal travel.
No. As I stated, pedal is still low with calipers removed and a 1 inch steel plate between the pads.
How are you blocking, clamping off front calipers? Could hoses be ballooning?
clamping the hose an inch or so before the caliper. Leaving as much of the hose in the system as possible. As stated, they have been replaced.
You did not give any vaccuum info for the manifold vacuum (both in gear and in park), vacuum past one way valve, ability of the booster to hold vacuum etc. The only variable appears to be engine speed. Some testing with a vacuum gauge may show a problem.
I forgot to say that, yes, the booster maintains vacuum for one brake depression, after engine has been shut off for any extended time. I have done so many checks on this, that I'm sure I have missed something in my post. Although I didn't gauge it, vacuum is highest at idle, in park, and cruise with no load, foot off gas. Since the low pedal doesn't re-occur with high vacuum (cruise no load)…
Steve, an intermittent low pedal is not likely air. Air should be consistent. It sounds more like a bleed off of fluid somewhere intermittently. I think of a leaking ABS HPU valve allowing bleed off into the accumulator.
I agree. I already tried 2 used hcu's on this. But…aren't those same possibly leaking valves still pressurized when the front hoses are blocked, and pedal is good?
When I have hydraulic brake issues that stump my thinking, I stop and retrench. What is the complaint, what are the possible causes and how can I prove things to eliminate them as being a problem until I am left with the culprit. In this case, based upon what you have posted, I would be putting pressure gauges onto each wheel and starting up the engine and stomping on the pedal. None of the…
Retrenching is why I'm on this thing. My normal diag is leaving as many questions as it is answers. I'm creating more diag methods as we go! I don't have access to those pressure gauges, but I am familiar with them. That could help to see what is happening. Great idea on the stethoscope. Worth a listen! Thanks!!
Right on, Obie. I've seen engines with worn timing chains cause brake assist problems…
… IMHO ... calipers should not “stretch”… not if they are made of GOOD metal/castings. … Dan H. … Hobbs … South Texas …
I agree. Of course I didn't make them. And it would be odd to have 2 start stretching at the same time, and one reman caliper having the same issue. And it not be a common issue on ANY ‘silver bullet’ group….
The amount the casting is giving, when the brakes are applied, as measured between the casting around the piston, and the outer pad casting. Obviously there shouldn't be any, but these calipers do have .030". Not sure it is the correct term, never heard an official reference to it.
That's a new one for me! If the stretch, flex, spread, what ever it's called :) more than say .030, where's the problem, the caliper casting, mounting brackets ?
You did say casting, trying to visualize mentally what's happening here, not working:) Need to see a video of it I guess :)
This is not the correct caliper, but it tells the gap I am measuring. Hope this helps.
I let this post slip my mind, sorry about that. Thanks for the photo, if the distance increases more than .030 it's pretty much a defect? Thanks again
Since there are no AM caliper castings from new, the castings are all OEM, and are not part of a rebuild. Extremely unlikely for this to be anything related to the issue. Also, I'm not too sure that you've got a problem here, if the vehicle stops normally. A check of the pushrod length may help, but I'm not seeing any real issue, from your description. Calipers are ‘self-adjusting’ so it's…
Mark… That term was first used by Darren McQuaid I believe… I would have to see/measure it … Hobbs.
Dan it was actually the OP that used it originally and since I recognized immediately what he was saying decided to not confuse him by changing it to “spread” or“flex”
with all due respect . the braking system seems to operate as prescribed, I personally would inform the customer , that all is as should be, find some brake literature and give it to her to read, her heavy duty boyfriend is your main issue…
Yes and no. I doubt I'll find any documentation stating it's normal for a pedal to sink KOER, until you start to move. At the shop this is referred to as ‘Chevy soft’, and normal for some vehicles. At least in our experience. And yes, I think the boyfriend is a factor.
We have had 2 of these, both had been around the block already and parts cannon shot at them. 1 with a low pedal and 1 after driving for long periods the pedal got rock hard and the car wouldn't move. Both required a booster rod adjustment. We adjusted the rod and set so when holding the master cylinder tight to the booster by hand unbolted the person holding the master cylinder would feel the…
Adjusting the booster rod was a thought I had. When bench bleeding the master, (I felt this on all of them). There was more movement (say about 3/8'") before the shaft stopped. More than I am used to, but all the replacements felt about the same. Do you recall if the low pedal symptoms I described, where present in your low pedal car? could you put any kind of number on how far the piston…
The pedal dropping 1 ½ inch when removing the clamps up front sounds like pistons are retracting back into the caliper, rust in the lip the dust seal is in causing a spring back effect maybe or just the seal not letting the piston slide but pulling it back after the braking from old, contaminated fluid. Checking, and, or adjusting the rod doesn't take that long. Neither does popping a caliper…
Had to sleep on some of this….I don't think piston retraction can be the issue with this one. Key symptom is KOEO, pedal is high and hard. Repeatedly. No low pedal. WITHOUT releasing pedal, start the car. Pedal IMMEDIATELY drops way too far. The pedal has never been released for the piston to retract to cause the low pedal. If I am incorrect on this, please teach me.
One point I will remind you of on the pedal with booster is the force of application is much higher. That extra rise in pressure may be doing something naughty? Remember, the fluid is not significantly compressible at these pressures. I think you can only be moving more fluid somewhere or the booster is somehow compressing the linkage by distortion of the firewall. Or some of each. All of the…
Do you recommend these: smile.amazon.com/IPA-Tools-Inno… Or something like this: smile.amazon.com/SSBC-A1704-Bra…
Steve, I would like to chime in on these. I have both, each has it's advantages and dis advantages. The direct reading ones (bleeder screw) have the advantage of testing in-situ. This allows you to see whats going on not just with pressure readings in the system but also allows you to monitor in place drag/binding of the mountings, pads etc. And since you will not be moving the caliper allows…
Steve, I am a fan of attaching to the bleeder. I am doing my testing with an assistant. I am bleeding the pressure gauge. I use at least two at a time. The first one has certain ease of use without mess but will not help on a drum brake.
Thru the years..most Chrysler cars have fooled us with low brake pedal (bleed brakes forever) on drive lift …we now drive it regardless of how pedal feels..on road tests pedal feels normal n brakes stops pefect..come to a stop light u can make pedal feel soft by pushing on it..so i think its characteristic of Chrysler cars?? just my opinion..ive fixed alot of GM's with low or no brake pedal by…
Agree, must actually drive it, you don't know how pressure normal braking is going to require.
I feel your pain. Bringing low brake pedal problems to the customers attention never seem to end up good. Had a similar nightmare last week when our lube tech put low pedal on his inspection sheet. Sounds like you‘ve narrowed it down to front calipers and Travel distance from resting, but is it truly a problem? Boyfriend has probably never driven the car before and now since it isn’t as good as…
Does the scan tool allow isolation valve activation within the abs hydraiulic unit? Thats how i check the abs hyrdaulics, by isolating individual circuits using the scan tool, and see which circuit brings firmness back to the pedal. I suppose its possible to fabricate block off caps to achieve that same result, but some abs units are buried too farto make that practical.
I don't know and am not familiar with this procedure. I will check into this.
I found the screen for commanding the separate inlet and outlet valves, this is what you are referring to? Can you explain your process of isolating the circuits? Thanks.
Sure, each wheel/circuit (front, rear, or however its split) should have an isolation valve command and dump valve command. Command the L/F Isolation solenoid closed (on) and recheck the pedal travel. Did it rise, or get more firm after closing that valve? If it did, you found your problem side. If not, move on to the R/F and so on. You should hear the ABS pump cycle on and back off between…
Thanks. This unit has inlet and outlet valves for each corner, and the ability to cycle each one on & off, individually. No isolation valve listed. it does tell whether the valves are normally open or closed. Good idea with a different vacuum source!!
Steve, I've been down the “low brake pedal” rabbit hole more than I like to remember. Since buying and using this Thexton 803P block off kit, it has helped with isolating the brake circuit problems. These plugs work also on the ABS HCU where I have used them the most since purchasing. HTH
First off I’d like to thank all of you for your input on this mess. The short version: replaced the brake booster. The long version: After considering and checking many of your suggestions, I unbolted the master cylinder and checked the amount of pedal movement before the m/c started to move. It was a couple of inches. The push rod is adjustable. Ended up making it about ¼ in longer. The pedal…
Hello, Steve. In my long experience, master cylinders are most difficult to diagnose for low pedal or low vacuum assist complaints. My last go-around was with a vacuum booster that had a ½" hole drilled through the back of it. The symptoms were that the electronic throttle would go into limp mode and I kept getting a random “low vacuum” code. We then noted that the harness coming off the…
I agree, low pedals and boost issues can easily become nightmares!! We've seen so many that we have an unwritten check list, which TYPICALLY catches the issue. In my many years, I have NEVER seen a booster do anything like this! I can't believe you didn't have idle issues with your half inch hole!!