Odd Brake behaviour. (help needed)

[YBTURBO]

ok so this is a bit of an odd one and im hoping one of you have come across it at some point before i start taking things apart in my limited time.

regular driving car (brakes) feel fine they slow to a stop just as you would expect them too pedal dont feel too hard or spongy or soft. but if you try brake heavy the brakes will initially grab hard but the pedal feels like it jam's half way down and the brakes fade. still stopping the car but by no means quickly.

tried searching about and only thing i have found that has come close was fixed by adjusting the rear shoes! but no actual info on why or how.

so any help or insight would be great guys.

car is a 1.4 fiesta duratorq 2007

[isetta]

I don’t know for sure but I am wondering if it’s a problem with the servo or the vacuum pump or the vacuum pipe that runs between the two. When you say it jams halfway down and fades I’m wondering if that indicates a partial vaccuum leak .  I think but not sure that vacuum pump is on the end of cylinder head in the battery direction.

[YBTURBO]

my explanation isnt the best by it jams and fades what i mean is the brake pedal stops and does not move any more and the brakes done seem to slow the car as much. its hard to explain but its proper got me confused. pumping to a solid pedal engine off then starting the pedal will drop but again to half way then it go's nowhere. if im on a steep ish hill i have to really push on the pedal to get it to hold though so not really sure where is best to start with the damn thing.

[DaveT70]

Possibly seized rear drum brake adjusters, broken/weak drum springs or worn rear shoes or possible leaking rear cylinders

Release the adjustment on the handbrake cable and remove the cables from the drums, then repair the drum/shoe adjusters, then re-fit the handbrake.

Don't re-adjust the handbrake cables until you know the auto adjusters are working (drive the vehicle in reverse, slowly, and brake hard until you get pedal feel back) 

Re-adjust handbrake cable

[YBTURBO]

3 hours ago, DaveT70 said:

Possibly seized rear drum brake adjusters, broken/weak drum springs or worn rear shoes or possible leaking rear cylinders

Release the adjustment on the handbrake cable and remove the cables from the drums, then repair the drum/shoe adjusters, then re-fit the handbrake.

Don't re-adjust the handbrake cables until you know the auto adjusters are working (drive the vehicle in reverse, slowly, and brake hard until you get pedal feel back) 

Re-adjust handbrake cable

thanks for the info i only get a day a week to have a chance to look but i'll get on that first chance i get and let everyone know how it went. (saturday is when it will be as thats the only chance i get lol)

[YBTURBO]

ok so update on the brakes the rear adjusters look screwed but the shoes and drums look ok, manually adjusted them and they seem to be working well and holding with the handbrake and cant be turned by hand with foot brake on. however the pedal still seems to be hitting a physical stop, light braking feels fine, slowing down from speed seems fine but if you try slam on to do an emergency stop it seems to work really well at first then it go's to a faded slowing rolls to a stop. hopefully i'll have a chance in the morning to check the front disk's and pads but need to work out whats going on with the brakes normal driving is fine but an emergency stop would never happen so not ideal.

 

any ideas on where to look next would be awesome guys

as always thanks for any advice.

[Tdci-Peter]

On 12/14/2021 at 11:17 PM, YBTURBO said:

is the brake pedal stops and does not move any more and the brakes done seem to slow the car as much. its hard to explain but its proper got me confused. pumping to a solid pedal engine off then starting the pedal will drop but again to half way then it go's nowhere.

I am with Isetta in suspecting a servo / vacuum problem. Pumping the pedal with engine off will remove(*) any vacuum in the servo, and the brakes will fell very hard on the pedal, but will not be applying much pressure at the disks & drums. Start the engine and the pedal will drop as soon as vacuum builds up, it will also feel quite a lot more spongy. This does not necessarily mean there is air in it, there is flexibility in the whole system: Flexi-hoses, brake pad compression, and particularly with disk calipers, there is a surprisingly large amount of flex in the caliper itself. The much greater hydraulic pressure you get with the servo will show up this flexibility, while it will feel solid with the pathetic pressure you can get manually.

If the pedal goes hard halfway down its travel, then most likely it has run out of vacuum, and the servo is not providing any further assistance.

It is possible there is a problem with one axle, but if so, I would suspect the front, as that provides the majority of the braking. I believe there is a safety point in the pedal travel where the pressure to each axle becomes separate. This means that a pipe burst or sudden major brake failure will not result in all 4 brakes being lost, only one axle. But if the braking past this point feels really feeble, I think the problem must either be in the front axle, or in the servo / master cylinder.

I had something like this once. Approaching a roundabout at the end of a dual carriageway, while braking there was a loud bang, and the pedal shot right down. I am sure there was still some braking, as I had quite a bit of speed to lose, now rather urgently! I did manage to release the pedal after a few seconds, and re-apply, and got back to more normal braking. And just about slowed enough, but it was very scary. After the roundabout, I pulled over, and tested the brakes, they seemed fine, so I continued the journey home very cautiously, after I had stopped shaking! When I inspected it, one back disk pad was worn rather thin, and another was missing entirely! The pad had detached from the backing plate, and was thin enough to shoot out through the narrow gap between caliper / bracket and the disk. That was the bang I heard, and the caliper piston movement needed then allowed the pedal right down.

This was on my previous car, a Vauxhall with horrible rear brake set-up: tiny drum handbrake inside a rear disk, and a hydraulic union had to be disconnected to properly inspect or change the rear pads. Yuk. My Focus is easy to maintain, and brakes are fully serviced every year now!

* Note: Yes I know that, pedantically, a vacuum is nothing and can not be removed. But mathematically, negative and relative quantities as just as valid as absolute ones, so as long as it is known and accepted that a "servo vacuum" is where part of the normal atmospheric air has been removed, the term vacuum can be used as if it were a real, positive thing!

[YBTURBO]

32 minutes ago, Tdci-Peter said:

I am with Isetta in suspecting a servo / vacuum problem. Pumping the pedal with engine off will remove(*) any vacuum in the servo, and the brakes will fell very hard on the pedal, but will not be applying much pressure at the disks & drums. Start the engine and the pedal will drop as soon as vacuum builds up, it will also feel quite a lot more spongy.

thanks for the info i'll try the servo next will have to get my hands on one first though, i'll also get round to taking off the front wheels and inspecting the calipers and pads. the pedal will go solid when pumped with the engine off but does drop when the engine is started, however the thought it might not quite have the vacuum to do the job fully does make sense so i'll go for that first. 

as always when i get chance to do anything i'll report back any findings or anything done and what the outcome is/was.

[isetta]

I don’t know what the vacuum pump is like on your car. On the  1.8tdci diesel there is a known problem where the rivets holding the pump together get loose after many years. The cure for that is to drill the rivets out and use nuts and bolts in their place. But I have never inspected a pump on your type of engine so it might be a totally different design which is likely as it’s much newer than the ancient 1.8 Diesel engine design

[Tdci-Peter]

2 hours ago, YBTURBO said:

i'll try the servo next will have to get my hands on one first though

On the Focus, the a/c pipes block access to the servo & master cylinder, making diy work on these rather impractical (unless the a/c is already empty!). I suspect this is true of many cars. So eliminating brake problems at the wheels first is a good idea even if they are not prime suspect.

A servo system in good condition should be able to give 2 or 3 full brake applications with a recently stopped engine. And there should be a distinct change in pedal feel when it does run out of vacuum. From the normal (engine running) slightly yielding feel to solid. Also it should be able to operate after a few minutes from stopping. Leaks through the diaphragm may be testable by holding light pressure on the pedal, moving it very slightly at intervals, and seeing how long it takes to go hard with engine recently stopped.

While the footbrake is off, vacuum is applied to both sides of the diaphragm. When the brake is applied, one side of the servo is first isolated from the vacuum inlet (actually an air outlet!), then air is allowed in to that side to create the pressure over the diaphragm to assist the brake pressure. This air going out to vacuum pump and in from atmosphere is regulated by a balancing valve that controls the hydraulic pressure to a set multiple of the pedal force. So with engine off and steady brake pressure applied, a diaphragm leak will rapidly remove the vacuum from the other side not being controlled, so losing servo assistance.

Listening for hisses from the vent port (hidden on the back of the servo somewhere) can also give clues, but usually needs 2 people who both know what they are doing. A rare luxury for me, I don't often have even one person who really knows what he is doing on many of the jobs I try for the first time!

[YBTURBO]

8 hours ago, Tdci-Peter said:

Listening for hisses from the vent port (hidden on the back of the servo somewhere) can also give clues, but usually needs 2 people who both know what they are doing. A rare luxury for me, I don't often have even one person who really knows what he is doing on many of the jobs I try for the first time!

im one of them people who at this point DONT know what they are doing but im also the sort that will find a way to learn and wont give up on sorting the issue. and im sure with the help of you guys i'll get to the bottom of it and when i go it will be well documented on here for others it may help in the future. just like the injection wiring when i done that as best and as clear as i can things will be written up for others to get help from. this needs sorting soon they dont seem to be getting worse but they for sure aint right and i need to get back to working out all the engine wiring that was my initial project. and i suppose i should free up the EGR soon too as its now stuck not sticking. but that weather permitting i will do over the xmas when i have 3 days off.

[YBTURBO]

ok so update time and today i have 2 things to report, first off the for those of you who know my many issues the EGR is now free and working again. ok now time for the brakes it was a split in the main servo vacuum line. should be an image with this post showing the split for now it has a 3 tape repair but new pipe is on its way. for anyone interested the temp fix consists of a wrap in isolation tape, then a wrap with self amalgamating tape (normally used to fix water leaks) and finally a wrap with loom tape as that has little to no stretch.

has it worked for the fix yes. should it be done and kept as a fix NO. if you have an issue like this and sealing it with tape sorts the problem BUY A NEW PART do not think the tape will become a perm fix.

[isetta]

Glad you found it and it is a simple fix, well simpler than it needing a servo or vaccum pump, but what wore through it and why?  Had the pipe come way from a clip so it rubbed on something it should have been held away from.  As this is a vacuum pipe and not one carrying liquid or air under pressure I think the repair would probably last a long time. I’m not saying don’t replace it but I wouldn’t say don’t use it.

[YBTURBO]

6 hours ago, isetta said:

Glad you found it and it is a simple fix, well simpler than it needing a servo or vaccum pump, but what wore through it and why?  Had the pipe come way from a clip so it rubbed on something it should have been held away from.  As this is a vacuum pipe and not one carrying liquid or air under pressure I think the repair would probably last a long time. I’m not saying don’t replace it but I wouldn’t say don’t use it.

it had come from the clip holding it to the bulk head and was rubbing on what im guessing was the back of the air box. its now patched up and back in its clips and your correct it would prob last without a problem but for the sake of less than £50 is it really worth waiting to see how long a "fix" on brakes will last!