740 brakes

petershen1984 · Post by **petershen1984** » 11 Apr 2004, 05:14

Hi All,

My brakes work (!!), but there's a wooshing noise inside the cabin when the pedal is depressed. Also, the brakes aren't very sensetive compared to new cars -- the pedal seem to go deeper before I feel the braking effect. I've flushed the brake fluid but the car still drives and stops the same. I don't think my car will have emergency braking.

Brake booster runs fine, should I check the master cylinder? thanks.

(1988 Volvo 740 no ABS).

Peter

petershen1984 · Post by **petershen1984** » 11 Apr 2004, 09:34

Sigh, I just realized that my cat converter may be gone too....

Volvord · Post by **Volvord** » 11 Apr 2004, 11:27

A "wooshing" sound (like the description) when you depress the brake pedal is usually a sign of a leaking brake booster. When you initally step on the brake pedal a "reaction disc" in the booster allows the vacuum to act on the diaphram which provides the boost assist, if there is a slight hole or tear in the diaphram you will hear this as a vacuum leak inside the cabin, often the tear or hole is not large enough to have a noticable effect on the booster function, but over time things will worsten and brake assist will be effected.

Converter restriction or plugging is also a common item on cars of this age, both mine have needed replacement

petershen1984 · Post by **petershen1984** » 15 Apr 2004, 23:20

Hey guys, I had my brakes checked out today. The mechanic says "some" pedal travel distance is normal for my non-ABS brakes. He then proceeded to replace the front brake pads -- yeah they were worn down unevenly, but still had about 1/3 remaining so I replaced them early. It does improve the pedal feel!

And they told me not to worry about the booster vacuum leak until it gets more serious (the booster costs around USD 200)

They said that ABS brakes feel sensetive, as a side effect of having the pump pulsating at 60 Hz or faster, depending on the "velocity" of pedal travel. Ah well, my 740 will live for a while..

Oh, and my car doesn't have a cat (meow~~!) converter to start with!

Volvord · Post by **Volvord** » 16 Apr 2004, 20:01

petershen1984 wrote: They said that ABS brakes feel sensetive, as a side effect of having the pump pulsating at 60 Hz or faster, depending on the "velocity" of pedal travel.

I am glad everything is good but what the mechanic told you about the ABS is "Bull Shit." The Volvo ABS is a supplementary system than retains all the characteristics of a standard braking system and only comes into effect when the wheels enter a "lock-up" or deceleration mode that triggers the ABS ECU. When the ABS comes into effect then the system cycles at 60 Hz. but when ever the system is not activated, which is 99% of the time then you would not have any cycling.

petershen1984 · Post by **petershen1984** » 17 Apr 2004, 05:05

that makes sense; if ABSes were always activated then you'd feel the pedal feedback (of the pulsation). Maybe that's why in an emergency, inexperienced drivers panic when the pedal vibrates and end up pumping (that 1% of time).

He probably had to say something to explain why the Lexus RX 300 drives so differently.

ozzimark · Post by **ozzimark** » 18 Apr 2004, 08:18

petershen1984 wrote:Oh, and my car doesn't have a cat (meow~~!) converter to start with!

..are you sure? i have an '85 740 gle, and it has a cat converter... and about the brake pedal going too far, i've driven two different 740's; mine and my dad's. his is a '90 /w turbo, but one thing i can never stand about his car is how far i have to push down the brake pedal compared to mine. i never was sure what caused it, but the brakes work fine. i'm not sure what will happen if you change the reaction disc (if it's possible to change just that...), but it'll probably change the characteristics of your brakes for the better, and the wooshing should go away.

Volvord · Post by **Volvord** » 18 Apr 2004, 09:20

Two things to keep in mind when comparing brake pedal travel. The first thing is normal travel, all new cars use low drag caliper which means the seals used in the brake caliper pistons are designed to pull the pistons (and subsequently the brake pads) back and away from the rotors. This reduces drag and increases the fuel economy. Pedal travel increases slightly in this case because of the extra distance the brake piston has to travel. Car engineers have addressed this concern with different designs of master cylinders but there is still more travel than the older vehicles.

The second reason for extra travel would be air in the brake lines, this is not a normal situation and the brake system should be repaired and the lines bled. One area in a car that is often overlooked is the brake fluid. Brake fluid is hydroscopic which means it normally and readily absorbs moisture right out of the air. As the moisture content of the brake fluid increases the boiling point decreases substantially. Also moisture in the brake fluid will cause corrosion to brake components, it is especially damaging to ABS systems. You should have the brake fluid flushed and replaced every 3 years.

petershen1984 · Post by **petershen1984** » 20 Sep 2006, 00:52

ozzimark wrote:
petershen1984 wrote:Oh, and my car doesn't have a cat (meow~~!) converter to start with!
..are you sure? i have an '85 740 gle, and it has a cat converter... and about the brake pedal going too far, i've driven two different 740's; mine and my dad's. his is a '90 /w turbo, but one thing i can never stand about his car is how far i have to push down the brake pedal compared to mine. i never was sure what caused it, but the brakes work fine. i'm not sure what will happen if you change the reaction disc (if it's possible to change just that...), but it'll probably change the characteristics of your brakes for the better, and the wooshing should go away.

First of all, my kudos to Matt for maintaining every single thread that has been posted this forum - I looked back for this post now that I have my newer 960 and wondered about the same brake problem, and Volvord's reply about low-drag caliper really explains what I feel.

Well, this particular 960 has a larger diameter booster (I wonder if it's compatible with the Allied Signal from the 740/940). No vacuum leaks to begin with, so there's not a need to replace that as a source for brake travel). Of course, this 960, being a US-spec, is fully loaded with ABS and Child Booster seats.

I noted the further pedal travel on this car, which takes some getting used to. But also, if I "manually repump" the brake a second time, the pedal travel is not as great. But then, I still want to upgrade my front pads and see about the more refined booster design. (My 740 had a new brake booster two years ago).

On an open strech of road, I made sure I had no other cars following me and simulated panic braking for ABS function. I can stop more quickly and discreetly (sans screeching), and there is no brake dive (good shocks!).