Odd Brake Problem

[Rich in Vancouver]

Hi all,
I am having a strange problem with my brakes. I recently fitted new rotors with Green Stuff pads and good used calipers.
When I press on the brake pedal it travels part way down then the brakes come on with a thump. By that I mean there is a point in the travel wher it goes from no braking to heavy braking almost instantly.
The car is drivable but I have to have a very light foot to deal with this.
The brakes are stock but with braided hoses. I have bled them but get no air.
Things are fine at the wheels, relatively new rear wheel cylinders, front calipers are working fine so that leaves three components.
The master cylinder, the booster, and the proportioning valve.
I have a spare servo and it seems that it would be simple to swap it over, but have no idea of the condition.
Does anyone know how to test a servo? If I push on the pedal side rod the M/cyl side rod moves. I have tried sucking on the vacuum fitting of my spare servo (not as disgusting as it sounds[xx(]) but get no vacuum. This doesn't seem right to me.
I have a nasty feeling that the M/cyl is shot but with that being the most expensive component I don't want to replace it on a guess, and I have no idea how to check the proportioning valve, or if they ever fail?
Of course I could spring for a new uprated M/cyl and booster but the funds just aren't available right now.

Any hints?

Cheers,
Rich

1975 TR7 ACL764U

[PeterTR7V8]

How far have you driven the car on the new brakes? Have you bedded them in? Try disconnecting the vacuum hose to the servo & see if it is less on/off.

The proportioning valve reduces the rear brakes rather than strengthens the fronts - as far as I know.

[HDRider]

There is also a proportioning valve in the Master Cylinder that can be damaged if the balance switch is not removed prior to any brake bleeding. It is the white plastic part located under the front portion of the Master Cylinder with two black wires attached via a plug.

The only other time I have encountered a similar problem was with older rubber flex lines when the inner bladder was leaking but generally it is only one side at a time that is effected.

Edward Hamer
Petaluma CA

[saabfast]

The switch in the u/s of the master cylinder is for brake failure warning and , as noted, should be removed when bleeding to prevent damage. To test the servo run the engine to build up vacuum, the pedal should feel 'normal', going down so far before becoming hard with the brakes on. Switch off the engine and press the pedal a few times. A hiss may be heard and the pedal will get harder at a higher level each time as the vacuum is exhausted. Kits are available to rebuild the servo.
If the master cylinder seals are worn the braking effect gradually gets reduced as the pressure leaks back. Kits are available to rebuild it quite cheaply but I had problems as i could not get the small piston (which operates the failure switch) out and a replacement recon unit leaked. I ended up getting an uprated M/C and servo unit from S&S which was easy to fit and gave improved brakes.

Alan
Saab 9000 Stg 1
Saab 9000 2.3 FPT Auto
'81 TR7 DHC

[Rich in Vancouver]

Thanks for the input guys.
The paperwork that came with the Green Stuff pads say no bedding is required, just drive gently for 100mi. Maybe I should try bedding them in the old fashioned way.
The hoses are all braided, so it's unlikely they are the culprits.
It sounds unlikely the booster and the prop valve are the culprits, maybe the Master cylinder?
I will give the car a good going over on the weekend to see if there is anything I have missed.

Cheers,
Rich

1975 TR7 ACL764U

[FI Spyder]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Rich in Vancouver</i>

When I press on the brake pedal it travels part way down then the brakes come on with a thump. By that I mean there is a point in the travel where it goes from no braking to heavy braking almost instantly.
<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">

A conundrum. If we look at boost...if the pedal is hard then is easier to press down with braking action increase suddenly, that would be consistant with boost coming on suddenly, usually the reverse of what we see. (sticky one way valve?)

I can't see this being caused by proportioning valve even if i was inoperative. The secondary piston in the master just actives the warning switch and even if it was out of position or frozen it would not effect braking. (Mine was frozen but was able to lever it out with screwdriver against cam surface visible in the switch hole.)

If you can't get vacuum on spare booster it's faulty. There a rubber diaphragm inside that can leak assuming the leak isn't coming from rusted pin hole in metal. It does come apart to replace diaphragm but it's difficult to twist open and compounded byt the fact it will fly apart because there's a big spring under compression in there. Be careful.

There conceivably be some kind of mechanical jamming of disc pads or a binding then abrupt release resulting in sudden braking.

"If I push on the pedal (side rod?) the M/cyl side rod moves"...If by the metal side rod you mean the differential brake warning rod, it shouldn't move. The switch rides on a narrow cam surface and if it moves the switch follower drops (raises as it's upside down) and closes circuit to turn on light in dash. Overhauling master cylinder is cheap assuming the cylinder isn't corroded which would wear rubber on piston and leak fluid (down the firewall to eat the paint so it will rust the metal beneath.

I would take it apart to see if it can be overhauled. If corrosion is just starting you might be able to hone it a bit. At any rate it's like cancer, better to catch it early.


TR7 Spider - 1978 Spifire - 1976 Spitfire - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra

[Rich in Vancouver]

The "If I push the pedal side rod....." referred to the
spare booster so no connection there.
I'll let you know how things go on the weekend.

Rich

1975 TR7 ACL764U

[Hasbeen]

How far down does the pedal go, before this happens?

If it is going further than usual, I think perhaps you may have a
slug of air in the valve, that moves suddenly.

I'd give it hard run, in a safe place, then another bleed.

Just thinking sideways.

Hasbeen

[Rich in Vancouver]

Good thinking Hasbeen!
I'll give it a hard run and will try to get it sideways as per your instructions![:D]
I'll also try bleeding it at the valve.

The pedal does go lower than usual but when it engages there is a really strange sensation, almost like a rubber diagphram turning inside out. I don't know if that makes sense but it feels much like when a rubber toilet plunger turns itself inside out while plunging.[:0]

Cheers,
Rich

1975 TR7 ACL764U

[Hasbeen]

Rich, Is the rear hose also new, & braided?

Thinking straight now.

Hasbeen

[Rich in Vancouver]

Yep, all 3 are new, braided Goodridge hoses.

1975 TR7 ACL764U

[samco]

I have had similar to this before and the master cylinder piston was sticking. It resists movement and then all comes in at once, I cured it by stripping the master cylinder cleaning the bore and fitting new seals.
Job done.

If it doesn't fit you obviously need a bigger hammer.

www.metronewquay.co.uk

Check out my album

http://s163.photobucket.com/albums/t299/samco_bucket/

My youtube progress http://www.youtube.com/watch?v=4kMDqLU1N7Y




1980 Californian import converted to right hand drive V8. 3.9L 265 BHP

[Reddragon]

Rich

In your post you stated "Green Stuff pads and good used calipers."

Is it possible that one of the pistons is hanging up a bit? When you drove your car did it feel to pull one way or the other?

To check the caliper action find a board a little thicker then the rotor. Remove caliper, insert board between the pads, and with an assistant have them slowly push on the brake pedal while you watch the action of the caliper.

While doing this test also watch how your new hose function. New parts do fail to.

If nothing else this will eliminate the caliper as a problem area and you can work back to the master.


Dale
1974 TR6
1977 TR7 FHC

[Hasbeen]

Rich, is the problem there both with the engine running, & not
running?

With no engine you eliminate the booster as a possible source of the
problem.

Hasbeen

[Rich in Vancouver]

I tried a few hard stops today. The fronts lock up before the rears and brake evenly.
I am pretty sure that Hasbeen has it right; a big lump of air stuck in the proportioning valve. My hard stops didn't dislodge it so the next step will be to bleed the valve fittings.
That would have been done today but this morning I made the mistake of logging onto a family tree site and the day has just disappeared in a flurry of dead relatives. That stuff is almost as addictive as old British cars!

Cheers,
Rich

1975 TR7 ACL764U