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'99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
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rollin_home

'99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by rollin_home » Mon Mar 02, 2020 8:49 am

Hi ,

I have a 1999 S70 AWD with a turbo and a IPD turbo control valve.
The problem is that after accelerating or opening the throttle, the brakes are very hard for about 15 seconds.

The first assumption is that the brake booster check valve is leaking off the vacuum but I connected a MightyVac hand vacuum pump to the brake booster hose and after raising the vacuum to 17 in of Hg, there was no leakage. I applied the brake pedal and the vacuum dropped by 1/3 but then held. Does this prove that the booster and the check valve are NOT leaking or am I missing something.

Two other thoughts I had were:
1) the IPD Turbo control valve is stealing too much vacuum and the brake booster is not getting the vacuum it requires to hold and be effective after open throttle (zero vacuum) conditions.

2) I dont actually know the amount of vacuum the car generates to the booster, it may be very low due to a vacuum leak elsewhere in the system. I made a DIY smoke test mechanism and will try and see if I can find a vacuum leak.

The brake booster is quite rusted on the bottom but that may. be normal and not necessarily a sign of leaking , picture attached.

Any thoughts or suggestions on this ? The car is unusable until fixed.

Thanks !

A37CAC52-ED7D-411D-BF58-732A931E40B2.jpeg
A37CAC52-ED7D-411D-BF58-732A931E40B2.jpeg (156.74 KiB) Viewed 156 times

scot850
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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by scot850 » Mon Mar 02, 2020 11:33 am

I run an IPD TCV on my 2000 'R' and have had no issues with brakes, but then I am seldom (read never) braking while accelerating?? My car is modified and pulls up to 20psi through the TCV.

Either way, even after accelerating, I have never noticed an issue with a hard pedal. Only time I have got that is when the car is cold and backing out of the garage and that is very seldom. I am just not ready to get into replacing the check valve on the booster front due to the limited accessibility. There is another in-line valve which again I have not changed out yet.

Checking the pipes for leaks or splits is a good thing to do as the main feed pipe can get porous near the manifold area where it attaches in particular.

Neil.
2000 V70 R - still being an endless PITA
2006 XC70
2003 Toyota 4Runner V8 Limited
2015 Kia Sportage EX-L
1993 850 GLT -Sold
1998 V70 XC - Sold
1997 Volvo 850 SE NA - Went to niece in California - Sold
2000 V70 SE NA - New project and test bed - Sold

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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by abscate » Mon Mar 02, 2020 11:47 am

That gnarly looking brake booster - I have at least two spares I can let go cheap for the needy and reasonably for those of means.

The vacuum leak is probably check valve or hose related but that booster will soon be a rooster

Are you on my NY MA NJ DC business loop by chance?
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1999-V70-T5M56 2005-V70-M56 1999-S70 VW T4 BMW
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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by rollin_home » Mon Mar 02, 2020 2:02 pm

Today I removed the MAF and ran the smoke tester needs nozzle into the turbo air intake. The only leak was a small amount from where the 3” air duct attaches to the turbo.

I also put the vacuum gauge on the Y fitting that the the brake booster hose plugs into so as to measure the vacuum the intake is pulling....was 20 “hg.

Replacing the booster is probably going to have to be done sooner anyway. If you got a used one for sale , I’m interested !

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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by abscate » Mon Mar 02, 2020 4:47 pm

rollin_home wrote:
Mon Mar 02, 2020 2:02 pm
Today I removed the MAF and ran the smoke tester needs nozzle into the turbo air intake. The only leak was a small amount from where the 3” air duct attaches to the turbo.

I also put the vacuum gauge on the Y fitting that the the brake booster hose plugs into so as to measure the vacuum the intake is pulling....was 20 “hg.

Replacing the booster is probably going to have to be done sooner anyway. If you got a used one for sale , I’m interested !
Ive sold two from my stash but i think I have two left - I definitely have one on the parts 1998 but I have not checked that one yet.
Empty Nester
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abscate
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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by abscate » Tue Mar 03, 2020 6:43 am

Argh.

1999- takes the 9485170 so I’ll have to find my 1999 stash


-1998 shows 9438037 , no longer available

1993-1998 9438036 no longer available
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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by rollin_home » Thu Mar 12, 2020 2:45 pm

I just completed replacing the brake booster with a better (less rusted) used unit. Man, what a tough and physically demanding job that was...getting up behind the brake pedal, completely removing the ABS unit, a real workout but I got it.

After cleaning up, I took it for a test drive and the issue was exactly the same , in fact the brake may be even weaker but definitely the hard-pedal-after-open-throttle issue was not resolved.

I was wary about replacing the booster without being able to prove that it’s leaking and the root cause of the problem but thought what the heck it’s rusted and what else could it be.

But the guessing approach didn’t work so now I have to ask if there is anyone on the forum who has seen this specific problem ?

Thanks

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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by abscate » Thu Mar 12, 2020 3:26 pm

You have the 'ejector" type vacuum device on your car, from when we met, if I recall. Check all those hoses and clamps. Does it have a vacuum pump or is it just engine vacuum feeding the booster?

I seem to recall a thread on the bushing on the booster leaking - rare but possible - I will try find.

Smoke test on the manifold and check valve definitely in order.
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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by rollin_home » Fri Mar 13, 2020 10:32 am

Opps , forgot to connect the booster vac line to the manifold. Now I have decent low speed braking. However the old issue of hard brakes after accelerating continues.

Review——
I have a 99 Volvo s70 with power brakes run off intake vacuum, car has no vacuum pump. Brakes work fine when moving slow. After accelerating brakes pedal is very hard, feels like ABS is cutting in. After 15 seconds brakes are fine again.

I replaced the intake to booster vac line.

I replaced the booster with a used but better looking non rusted one, bled all wheels, replaced all fluid....results: no change.

Using a MightyVac hand pump, I pulled 17” hg on booster vac hose. This held for 5 min. Maybe this proves there are no leaks in vac hose, check-valve or booster.

I removed Check-valve and did the suck&blow test with mouth , check valve allows only one direction of air flow, the right direction.

With engine off, I get 3 ‘assisted feeling’ brake presses before pedal gets hard.
Then, with foot on hard feeling brake pedal (vacuum has been used up by prior test), I start the car ...... results: pedal goes down about 1”.

Given the above what can I do to isolate the root cause of this loss of braking after accelerating?

—Thinking out loud ——-
Could it have anything to do with the ABS system and the brake system is otherwise fine?


Perhaps when going faster than 15 mph , the signals from the wheel sensors is incorrect and causing the ABS to kick in and cause the hard braking ?

I’m clueless.

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Re: '99-S70: Hard brakes, intermittant only after open throttle, vacuum test on booster shows no leaking

Post by abscate » Fri Mar 13, 2020 12:31 pm

Oh, wait...complication.

You don't get any ABS until you first exceed about 15 mph and the system self-tests. You will hear a 'brrrrt' for about 1/2 second when that happens, thats Normal.
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