soft brake pedal

[EsperHamid]

Hello I see you've been dealing with this for a while. Here's what I just went through with my brakes when it was cooler out brakes felt fine sometimes rarely had to pump them but when it got 70-80+ outside they would sink/get soft every stop I would have to pump my brakes just to barely stop no visible leaks but it turned out I had a very slow internal leak on the master cylinder nothing visible though put on a new master cylinder everythings fine now and I'm not lossing any brake fluid like I was. I was lossing very little though and the bad master cylinder was causing my front right caliper to seize up from uneven pressure from what I believe is due to the internal leak. So here's what I would do if I were you.

@Kev1990, I have no leaks from the calipers anymore, after I have replaced the left front one. I guess it is something with the Cylinder/brake booster.

can it be fixed, can something be done to it ?

does replacing it involves any special tools ?

is the brake fluid I am using too thin for my brakes to feel soft ?

[Kev1990]

Well for one tell me what brand/dot fluid are you using?

Replacing the master cylinder is one of the easiest things I've ever done on this car! For me at-least can't say for anyone else lol but special tools umm probably flare nut wrench's but it isn't required it's just so you don't strip your brake lines.

What is your year and model of your cb7? If I were you i'd put a new right caliper on too and inspect the rotor that's on there currently plus check that everything is tightened to specs as in torqued down properly because that could be a part of your knocking if something is loose.

I would go with this master cylinder BTW it's a good price and it comes with everything you need. Before you go ahead and order the one in the link below though make sure it fits your exact model of cb7

http://www.amazon.com/Cardone-13-251...ywords=13-2518

If you decide to replace the master cylinder here is a link for help : http://www.youtube.com/watch?v=trotrlIG2sk or you can even find other DIY helpful videos online by searching on google

Now if it were the brake booster does your brakes ever get hard while driving with everything plugged in as it should like the brake booster hose? As long as your brakes don't get hard while driving I would say your brake booster is fine as long as your brakes get hard with the car off if you keep pumping it then your booster is definitely fine don't even mess with that.

If you were any wheres near me you could come to me and I could help you out. Don't see where your located though according to your profile also how does your brake lines look? Including your metal brake lines. Has your master cylinder ever been changed is your car a auto or a 5 speed how long have you owned it for? If you change your master cylinder and that does not fix your problem we can move onto the drum brakes if you have drum brakes that is.

[johnl]

turned out I had a very slow internal leak on the master cylinder nothing visible though put on a new master cylinder everythings fine now and I'm not lossing any brake fluid like I was.

Yes, an internally leaking MC will cause a soft pedal. Typically it will start off reasonably OK when you hit the pedal, then the pedal will slowly sink to the floor as pressure is kept applied. Fluid is leaking, but it's leaking back into the MC fluid reservoir, hence no fluid loss or visible leakage.

I was lossing very little though and the bad master cylinder was causing my front right caliper to seize up from uneven pressure from what I believe is due to the internal leak. So here's what I would do if I were you.

The MC won't cause unequal braking distribution. If you have one wheel locking up (or other symptoms of unequal braking side to side) then it's a problem with a caliper, or possibly a problem inside a brake hose.

If it's a caliper problem then one of them is seized (or partially seized). Most likely the sliding pins are not sliding, which will be due to corrosion of the pin(s), which is most likely caused by water getting in through a failed rubber pin boot (there are two of these little boots per caliper, one per pin). It's easy to check, just take the pads out and see how hard it is to manually slide the caliper casting in / out on the pins, it should be easy (and it should slide on both pins, though often only one is seized). Less likely but still quite possible, the piston could be seized.

If it's a hose problem then it will be due to internal delamination of the internal layer of the hose. Some portion of the layer becomes detached and acts like a 'flap valve', impeding fluid movement though the hose (and thus affecting pressure seen inside the caliper cylinder). The flap can either prevent pressure from getting to the caliper, which will lessen braking at that wheel, or it can prevent pressure escaping from the caliper, which will tend to lock the brake on (at least for a short time). Or, it can do both. The hose may look perfectly OK externally.

as you can go right to Pepboys get a fully rebuilt caliper for our cars for like 39.99

Bugger me sideways, that's an amazingly good price. Here in Oz there would be at least another $100 on top of that, and you'd still think it was a reasonable deal...

Forgot to mention I let my master cylinder go for so long that it was putting so much extra pressure on the right brake it actually worn down the pad a lot faster then the left and actually caused my right brake pad to fall off while driving

Classic case of seized caliper....

[johnl]

However, I concluded that the bleeding procedure was successful since the brake pedal became hard.

To me this suggests a seized caliper, i.e. that there is a substantially 'mushy' pedal with boost but not so much without. Of course it's normal for the pedal to be softer with boost than without, and for pedal travel to be greater, but only to a certain degree.

When one pin seizes (or both) it causes the caliper to distort as the piston moves outward under pressure (because the pins can't slide easily). In effect the caliper becomes a spring, deflected by piston pressure. Pressure on the pads becomes unequal (inside vs outside pad), with the pad on the piston side pushing harder against the rotor than the pad on the outside, leading to one pad wearing faster than the other.

The pads will also tend to wear with a significant to substantial taper along the pad, because one end of the pad will be pushed harder against the rotor than the other end of the pad (because one pin is sliding and the other pin is seized). Tapered pad wear (caused by pin seizure) is likely to be more end to end when only one pin is seized, but more inner edge to outer edge if both pins are seized (with more wear near the outer edge of the pads).

When piston pressure is released the 'spring' in the distorted caliper means that the pad clearance without piston pressure tends to be larger, so it takes more initial pedal motion to push the pads against the rotor (i.e. there is more distance for the pads to move before they fully encounter the rotor faces, not helped by any taper pad wear). Also, the backing plate on the pad can flex, acting in a springlike manner, flexing before the pad is pushed hard against the rotor (which is why you sometimes come across pads where the backing plate has become curved).

When the booster is disconnected, the pedal will require more force (leg) to get X piston pressure in the caliper pistons (normal), but also (if a caliper is seized) you may not be able to apply enough force at the pedal to cause as much distortion in the caliper, so less caliper flexure exists, so the pedal 'mushiness' may well be less, but more when the booster is reconnected and more springy flexure is occurring.

the problem is that the fluid level in the master cylinder is still the same and I notice each time I open the reservoir cap, 2 or 3 bubbles appear.

Still not fully bled I think, or some slight leak still exists..

[Kev1990]

Dam John 100+ more for a rebuilt caliper? Here's why I say it was my master cylinder causing the right caliper to seize up. I first started by replacing the right caliper because it was seizing then the left one as preventive maintenance used all new honda oem brake pad clips and caliper bolts straight from the dealer along with all new brake lines including new AKEBONO brake pads. Still after all of that! My caliper was still seizing on the right side then started to seize on the left side. So I spoke to one of my local mechanics as he said it is rare but possible that a bad/internally leaking master cylinder could cause uneven pressure to the calipers. Ever since I have put the new master cylinder in not one of the calipers have smelled burnt or too hot like they were when they were seizing. So I am pretty sure the new master cylinder resolved all of that as I put brand new calipers all new hardware and brake lines none of it was resolved until the new MC was installed. Oh and dealing with none working brakes driving 120 miles a night basically city driving is horrible LOL! But since the new MC was put in like I said no more seizing and great firm braking power.

[johnl]

Dam John 100+ more for a rebuilt caliper? .

Not rebuilt, new (I did think you meant new). Finding an over the counter rebuilt unit would be unlikely here, I doubt there is the economy of scale in the Aussie market for someone to be doing batches of these, for this car.

Here's why I say it was my master cylinder causing the right caliper to seize up. I first started by replacing the right caliper because it was seizing then the left one as preventive maintenance used all new honda oem brake pad clips and caliper bolts straight from the dealer along with all new brake lines including new AKEBONO brake pads. Still after all of that! My caliper was still seizing on the right side then started to seize on the left side. So I spoke to one of my local mechanics as he said it is rare but possible that a bad/internally leaking master cylinder could cause uneven pressure to the calipers. Ever since I have put the new master cylinder in not one of the calipers have smelled burnt or too hot like they were when they were seizing. So I am pretty sure the new master cylinder resolved all of that as I put brand new calipers all new hardware and brake lines none of it was resolved until the new MC was installed. Oh and dealing with none working brakes driving 120 miles a night basically city driving is horrible LOL! But since the new MC was put in like I said no more seizing and great firm braking power.

Glad a new MC fixed your problem. I'm surprised though. Not a typical MC symptom. Now I have to try and figure out just how a faulty MC could actually produce this symptom, I won't be able to sleep until I do...

[Kev1990]

I know! LOL It's 12:12am here still not sleeping yet but seriously the brake problems I was having were driving me crazy especially when the new/rebuilt calipers were put on and still seizing this car being my main car for driving 120 miles a night I put a lot of thinking into it LOL! Although I did have another symptoms besides the calipers seizing my brakes would randomly sink sometimes so months into it many nights driving with no brakes or barely any at all with calipers seizing I just said fuck it took 30 some bucks put new MC in lol. Oh and my old MC was a Nissin so I'm assuming either original or replaced by the dealer at some point either way it'd be surprising if the original MC lasted that long.

[Kev1990]

The next 4 weeks are gonna be crazy for me doing a bit of work on the car before the cold ass winter comes LOL. Planning on valve adjustment, new valve cover gasket although it's not needed, fuel filter, spark plugs, EGR valve/port cleaning, new inner and outer tie rods on each side although only the left is a bit worn, coolant flush, transmission fill/drain, alignment witch is covered under alignment warranty thankfully and maybe another oil change before it gets real cold out even though it was just done a few weeks ago along with a new exhaust put on right before winter as it's getting a bit rusty lol. Anything else I can think of before winter comes too. Hmm hmm hmm. Already have new air filter, did one transmission drain/fill, new distributor, new 02 sensor, all new coolant hoses every single one of them, cleaned out FITV/IACV, new brighter stock headlight bulbs, new all season tires along with all new brakes basically plus a new axle/wheel bearing on the right side, new battery witch I'm pretty sure is already taking a dump on me taking it to get tested today actually, new oil pressure sending unit old one was leaking oil. I'm sure there's more that I'm forgetting but this has all been done within the past couple months. Any maintenance items I've missed John that you can think of that I haven't done or am not doing please post it lol. Forgot to add I also installed new high quality battery terminals along with a new alternator as the old one was a bit weak.

[johnl]

Now I have to try and figure out just how a faulty MC could actually produce this symptom, I won't be able to sleep until I do...

Let's try this; for a caliper to 'seize' even when there is noting wrong with it (as appears to have been happening with your brakes after rebuilt calipers were fitted), it must be that pressure isn't able to escape from the caliper piston.

Something is preventing (or at least inhibiting) pressure release, and it will be elsewhere in the hydraulic system (since we're assuming a perfectly functional caliper). It could be a delaminating layer in the rubber brake hose, or debris somewhere in the system. It could be related to the MC.

Inside the MC bore is a small orifice that allows fluid from the reservoir to enter the cylinder. When the MC piston is pushed (by the pedal) the piston seal moves a very short distance before the seal passes this orifice, which isolates the fluid in the cylinder from the fluid in the reservoir, and allows cylinder pressure to rise (with increasing piston motion) without this pressure 'leaking' back into the reservoir (which is why an internal seal leak can cause fluid to leak back into the reservoir, and the pedal to fall with no external leak).

When the pedal is released the piston and seal moves back to a position where the orifice is again uncovered and fluid can move between the cylinder and reservoir. If the piston fails to move back far enough (for whatever reason) then the orifice may not be uncovered, and so the fluid remains trapped in the cylinder. Any existing pressure in the hydraulic circuit can't be released, so the caliper piston doesn't fully retract, and some pressure remains applied to the pads.

If the pressure in the hydraulic circuit can't escape into the reservoir and the brakes remain partially applied as a result, then heat will build in the brakes and cause the fluid to expand. The expansion of the trapped fluid will further increase the pressure, which still can't escape, and the brake pads will be applied harder, the brakes will get hotter, the fluid will get hotter, the pressure will increase further...

I recall a similar problem with my Accord when I failed to adjust enough free play in the pedal. I made sure there was some free play, but it was less than the specified play. This caused the rear calipers to start remaining somewhat engaged after several brake applications, the rear brakes started to 'drag', and the problem became worse and worse as I limped the car slowly back home (with smoky rear brakes). After they cooled the brakes were fine, short re-test drive and they were dragging again. After re-adjusting to give slightly more pedal free play the problem disappeared and hasn't recurred since.

Given this possibility, is it possible that your pedal free play was a bit on the tight side?