low enguagement brake pedal.. why?

[John Milmont]

So this is a problem I have been dealing with in some of the cars I drive. I have done all of the usual things and am pretty much out of ideas so i thought I would ask you guys since this forum seems to contain very knowledgeable people. The problem is that brakes don't come on until the pedal is really close to the floor, leaving a lot of travel and making the brakes just really crappy.

Now, i have bleed the brakes, replaced master cylinders, pads and everything with no improvement. I have even checked the booster for leaks and it was fine. On my RX-7, i have adjusted the master cylinder push rod and had some success; its amazing how sensitive that pushrod is to adjustment. It is threaded which allows you to adjust how long it is, 1/8th of a turn is the difference between having the brakes drag like crazy, and barely having any brakes at all. But I don't think i should have had to do this.

The only thing thats left is the rubber lines; these are pretty old cars, the rx-7 is a 1990 vintage. Do rubber lines just get old and start expanding massively?

 

[Jim Cowden]

John

Yes I would change the hoses anyway.
If you clamp one brake line at a time you can source the end the problem is steming from if it is a caliper or similar.

It may be the reaction pad in the booster.
Put a new one or a harder one in, if it is to hard it will be very sensitive and lock brakes easy so dont get to greedy.
Go the OEM new one first.

See how you go

Jim

 

[Chris Duncan]

Single piston calipers?

It sounds like stuck caliper pins.

that's really sensitive MS push rod adjustment, something's wrong. Usually not that sensitive. It's possible the clearance between the master and the booster is wrong. Did you check the depth of the replacement MS, compared to the old one? (where the push rod from the booster goes into the master)

 

[Paul W]

Its worth trying to bleed the brakes with one end of the car jacked up as air can just sit in some places and not escape.

 

[Dave P]

John:

Yes rubber coated lines expand a bit. Could you give a bit more info on the brake system. Does it have discs all around? Stock master cylinder?
does it have a stock proportioning valve?

Dave

 

[Randy V]

Yes the rubber lines will make a difference...
Even new ones are subject to expansion..
If you really want a high and tight pedal, look into replacing the lines with steel braided teflon lined AN3 or AN4 hose/fittings.. Also if your car does not have ABS, you can implement 2# Residual Pressure valves in the circuits. These will keep your pedal high and with no penalty.

 

[John Milmont]

Well, my rx-7 is undergoing a lot of work right now. i will put SS braided lines on it and take of the booster to inspect it. The problem has to be one of those 2 things. I read this link Reaction Disk pictures and walkthrough - HybridZ and even though i dont share all the symptoms this guy has, i do have some. This guy in the link says he can lock his brakes no problem, I have trouble locking mine sometime - unless the pushrod is adjusted so its too long, then it locks real easy! So maybe its a problem with the reaction disc like Jim C suggests; though, i think this is uncommon for rx-7's. I never knew about the reaction disc untill now!

The brake system is a stock RX-7 system, Disc's all the way around with 4 piston calipers in front. I rebuilt them some time ago. I have never touched the rears outside of bleeding, but they seem to work fine.

I have a old ford contour with exactly the same problem. Though, i don't think that car is worthy of SS lines... This car has drums in the rear, and i dont think the pad can be adjusted any closer to the drum.

I also have a '89 f-body (firebird) with a similar problem. But that car does have SS lines. Plus, on top of that, the engagement is always changing. Some times its high, other times its low. I have tracked that car and its down right scary at times. This car has disks all the way around with a wilwood proportioning valve, it also has aftermarket PBR calipers in the font. Though, it has had the problem long before the valve and calipers went in. It was dissapointing that the brakes were still crappy after the new calipers... Booster has been checked for leaks.

All cars are properly bleed and have had new pads/rotors with no improvement.

I wonder why i have so many cars that have brake problems!:huh:

 

[John Milmont]

Single piston calipers?

It sounds like stuck caliper pins.

that's really sensitive MS push rod adjustment, something's wrong. Usually not that sensitive. It's possible the clearance between the master and the booster is wrong. Did you check the depth of the replacement MS, compared to the old one? (where the push rod from the booster goes into the master)

yes, this is the pushrod im talking about. The one from the booster to the master cylinder. Yes, the depth of the replacement MS was exactly the same as the stock one.

Maybe the pedal to the booster pushrod should be adjusted?

 

[Dave P]

John:

The reason it is such a fine adjustment is that by adjusting the rod you are making the seal in the master cylinder closer to the cut off point.
Too far you get lock up because the seal blocks the fluid hole into resivoir thus preventing fluid return into the resivoir.
With the AP master cylinders they check every one for this. Some have shims to make the length proper. I would not bet on the OEM or after market being that accurate on this adjustment.

Hope this helps Dave

 

[John Milmont]

well, its not like the wheels are locking up by themselves when i adjust it. I would say it feels a lot better when its long, the pedal feel is high and the brakes seem like they have way more power. The problem is when its like this, they tend to drag. Also, they feel different at different times. At a stop, it will feel pretty much 'normal' but at speed, the brakes feel pumped up and they start dragging. I'm guessing when i have the push rod that long, its right on the boarder of activating the brakes and somehow they just come on.

maybe my 'reaction disc' is worn out or somehow too soft/long and its taking up too much of the pedal throw.

 

[Dave P]

John:

If seal partially blocks the inlet hole the fluid does not travel back to the resivoir. The system will have pressure in it and the brakes will drag. I have had this problem once on our bedding machine. It acts like the more you push the pedal the pressure increases and the drag increases.

 

[Jac Mac]

There will be two recovery ports in your dual master cyl- one for the rear & one for the front portions, you might have the freeboard reduced to the point where one never opens & therefore maintains residual pressure in that part, preventing proper bleeding of the system, I have also seen examples where rebuilt M/C have had the divider seal ( not the correct name, but it keeps the fluid from front & rear sections seperate ) on the rear piston( Front in cyl ) fitted back to front which effectively turns the whole system into one rather than dual.. Try the car on some smooth wet concrete from about 20 mph in a panic stop & have observers check all wheels for braking ( Exercise caution while checking).

Is the motor modified, the booster seldom functions as designed once manifold vac is altered

 

[John Milmont]

Now that i had some time to think about it, I'm totally sure that the 'reaction disc' is the culprit. It makes so much sense. All of the cars that im having trouble with are old, and like most rubber parts, i'm sure that reaction disk gets crushed over time. When this part gets crushed, it shortens the overall length of the pushrod. I figure thats the reason why the pedal feel gets close to the floor. I know the MC isnt clogged or anything, I have replaced it and nothing happend. It has to be this reaction disc.

 

[Paul W]

Not sure if this is any use but I have heard of the newer more modern brake fluids attacking the rubber seals in the system - I think i the uk dot 5 is the synthetic type that can cause issues - might be worth investigating.

 

[Jim Cowden]

Sorry did not read post correctly.
Deleted

 

[kiwi ka]

Now for some thing different, I am sure JAC MAC will know this trick, I call it the `old Anglia trick`. In the car I have just built, I fitted new 4 pot Willwoods on the front and second hand Audi 2 pots on the rear both driven by seperate 3/4 inch masters. The back brakes were hard as straight off, the fronts were just soft, allways requiring multi pumps to come up. Tried every thing, even wrote to Willwoods who said the calipers may be draining back..impossable i thought. I spoke with a `mature` mechanic friend and he said pump them up and jam them on with a broom handle, leave them for a day or so and they will be perfect. Bugger me they are. I had allways thought that the seals where kind of rolling back that little bit much and the continious broom handle pressure made them move out and stay out. Got real good brakes now.
Cheers

 

[Jac Mac]

Guess I dont know everything Russell , I have found that 'new' master cyl pistons will sometimes not return hard back against the circlip on first install & bleed which will leave the compensation port blocked, which in turn prevents proper bleeding , this could be the same scenario that your 'mature' friend was dealing with, plus it only takes a slight misalignment of caliper to rotor in the case of 4 pot setups to create a bit of 'spring' in the system & give a dud pedal and we have not even put any dynamic loads on the system yet.

 

[kiwi ka]

Jac Mac, it was the caliper pistons that were not staying out and `just` retracting causing the soft pedal. Hard as hell now.
Cheers

 

[Jac Mac]

Most if not all calipers are designed with the O-Ring grooves machined so that the ring is slightly distorted in section when fitted and therefore tends to pull the piston back while not in use to prevent drag. The larger the dia & number of pistons the bigger the problem is likely to be especially with small dia master cylinders.

 

[Jim Cowden]

well, its not like the wheels are locking up by themselves when i adjust it. I would say it feels a lot better when its long, the pedal feel is high and the brakes seem like they have way more power. The problem is when its like this, they tend to drag. Also, they feel different at different times. At a stop, it will feel pretty much 'normal' but at speed, the brakes feel pumped up and they start dragging. I'm guessing when i have the push rod that long, its right on the boarder of activating the brakes and somehow they just come on.

Bugger I still cant get that box to come up around the quote.

John I keep looking at the posts and more info comes out the reaction disk will not give a changing pedal height.
At speed the slightly dragging pads will generate heat into the calipers and fluid.
The fluid expansion will generate more pad clamp as the fluid has no were to go but push the pistons out this is why it is worse at speed, as it cools the problem goes away.
I do feel it has nothing to do with push rod clearance you are band aiding the problem just set it to manufactures spec.
If you back the master cylinder nuts off and hold the cylinder on the booster lightly,have someone apply the brake so the free play is taken up out of the pedal to booster,from that point on you are feeling the push rod clearance, when it pushes the master off the booster that is the amount of clearance you have.

Before you put the braided lines on clamp each wheel to narrow down the source.
You may have more than one problem John.
Check basics like wheel bearing adjustment ,this can give you pad knock off as the wheel loads the disk will push the pads back then on the next application the pedal will be low.

When you did the calipers did you use OEM kits or aftermarket.
I had a situation like Russel described due to the seal section not being correct.
Did the problem start after the cal were done or has it always been average.
Clamping the front lines will help determine this.
I am presuming the calipers are the original equipment or are they a retro fit and brackets have been made to suit if so I would be looking at Jac Macs suggestion of misaligned brackets.
If the Calipers are not original maybe they are a bigger piston and you need to go for a larger master ?.
This maybe over explaining it but sometimes a small piece of info is left out and you wont get the wright answer and I mean that in the nicest possible way.

Jim
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